MM-53125 Add feature to convert group message to private channel (#24421)

* Added convert to channel menu item

* WIP

* refactored channel name input field and created conversion modal

* style

* style

* WIP

* wip

* Created API to fetch common teams of GM members

* Added UI for all members deactivated

* Fetched common teams in client

* WIP

* Added a required attribute to DropdownInput component

* Fixed a case with dropdown input required flag

* WIP

* API first draft

* Genetayed layers and mocks

* Fixed create channel bug

* WIP

* Added cache invalidation

* Calling API from client

* Updated API to accept name and display name as well

* WIP

* Moved converted GM to correct category

* Style fixes

* Added logic to move user to new team/channel after GM conversion

* Prevented guest user from performing action

* Added loading indicator

* Added smoother height transistion when loading finishes

* UI imporvements

* WIP

* Formatted GM conversion message on client side

* lint fix

* Moved convert option from sidebar menu to channel header menu

* Some cleanup

* Updated server layers

* Fixed i18n

* Fixed types

* Fix server i18n

* Fixed channel creation bug

* Added store test for GetCommonTeamIDsForMultipleUsers

* Server tests done

* Updated snapshots

* Updated layers

* lint fix

* Update tests

* For CI

* lint

* restored debug code

* Used user ID instead of username in channel conversion post

* WIP

* Review fixes

* LInt fixes

* Test fix

* WIP

* WIP

* WIP

* wip

* Review fixes, lots of them

* Review fix

* Disabled WIP test

* test

* Cleanup

* Test fix

* removed testing line

* Fixed incorrect default message

* Review fixes

* Fixes

* lint and i18n fix

* Setting category on server side

* updated i18n

* Updated tests

* Added tests

* Refs cleanup

* added test

---------

Co-authored-by: Harshil Sharma <harshilsharma@Harshils-MacBook-Pro.local>

server/channels/api4/channel.go

@@ -60,6 +60,8 @@ func (api *API) InitChannel() {
 	api.BaseRoutes.Channel.Handle("/members_minus_group_members", api.APISessionRequired(channelMembersMinusGroupMembers)).Methods("GET")
 	api.BaseRoutes.Channel.Handle("/move", api.APISessionRequired(moveChannel)).Methods("POST")
 	api.BaseRoutes.Channel.Handle("/member_counts_by_group", api.APISessionRequired(channelMemberCountsByGroup)).Methods("GET")
+	api.BaseRoutes.Channel.Handle("/common_teams", api.APISessionRequired(getGroupMessageMembersCommonTeams)).Methods("GET")
+	api.BaseRoutes.Channel.Handle("/convert_to_channel", api.APISessionRequired(convertGroupMessageToChannel)).Methods("POST")
 
 	api.BaseRoutes.ChannelForUser.Handle("/unread", api.APISessionRequired(getChannelUnread)).Methods("GET")
 
@@ -2194,3 +2196,86 @@ func moveChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.Logger.Warn("Error while writing response", mlog.Err(err))
 	}
 }
+
+func getGroupMessageMembersCommonTeams(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequireChannelId()
+	if c.Err != nil {
+		return
+	}
+
+	user, err := c.App.GetUser(c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
+	if user.IsGuest() {
+		c.Err = model.NewAppError("Api4.getGroupMessageMembersCommonTeams", "api.channel.gm_to_channel_conversion.not_allowed_for_user.request_error", nil, "userId="+c.AppContext.Session().UserId, http.StatusForbidden)
+		return
+	}
+
+	if !c.App.SessionHasPermissionToChannel(c.AppContext, *c.AppContext.Session(), c.Params.ChannelId, model.PermissionReadChannel) {
+		c.SetPermissionError(model.PermissionReadChannel)
+		return
+	}
+
+	teams, appErr := c.App.GetGroupMessageMembersCommonTeams(c.AppContext, c.Params.ChannelId)
+	if appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	if err := json.NewEncoder(w).Encode(teams); err != nil {
+		c.Logger.Warn("Error while writing response from getGroupMessageMembersCommonTeams", mlog.Err(err))
+	}
+}
+
+func convertGroupMessageToChannel(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequireChannelId()
+	if c.Err != nil {
+		return
+	}
+
+	var gmConversionRequest *model.GroupMessageConversionRequestBody
+	if err := json.NewDecoder(r.Body).Decode(&gmConversionRequest); err != nil {
+		c.SetInvalidParamWithErr("body", err)
+		return
+	}
+
+	user, err := c.App.GetUser(c.AppContext.Session().UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
+	if user.IsGuest() {
+		c.Err = model.NewAppError("Api4.convertGroupMessageToChannel", "api.channel.gm_to_channel_conversion.not_allowed_for_user.request_error", nil, "userId="+c.AppContext.Session().UserId, http.StatusForbidden)
+		return
+	}
+
+	if !c.App.SessionHasPermissionToTeam(*c.AppContext.Session(), gmConversionRequest.TeamID, model.PermissionCreatePrivateChannel) {
+		c.SetPermissionError(model.PermissionCreatePrivateChannel)
+		return
+	}
+
+	// The channel id the payload must be the same one as indicated in the URL.
+	if gmConversionRequest.ChannelID != c.Params.ChannelId {
+		c.SetInvalidParam("channel_id")
+		return
+	}
+
+	auditRec := c.MakeAuditRecord("convertGroupMessageToChannel", audit.Fail)
+	defer c.LogAuditRec(auditRec)
+	audit.AddEventParameter(auditRec, "channel_id", gmConversionRequest.ChannelID)
+	audit.AddEventParameter(auditRec, "team_id", gmConversionRequest.TeamID)
+	audit.AddEventParameter(auditRec, "user_id", user.Id)
+
+	updatedChannel, appErr := c.App.ConvertGroupMessageToChannel(c.AppContext, c.AppContext.Session().UserId, gmConversionRequest)
+	if appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	auditRec.Success()
+	if err := json.NewEncoder(w).Encode(updatedChannel); err != nil {
+		c.Logger.Warn("Error while writing response from convertGroupMessageToChannel", mlog.Err(err))
+	}
+}

server/channels/app/app_iface.go

@@ -477,6 +477,7 @@ type AppIface interface {
 	CompleteSwitchWithOAuth(service string, userData io.Reader, email string, tokenUser *model.User) (*model.User, *model.AppError)
 	Compliance() einterfaces.ComplianceInterface
 	Config() *model.Config
+	ConvertGroupMessageToChannel(c request.CTX, convertedByUserId string, gmConversionRequest *model.GroupMessageConversionRequestBody) (*model.Channel, *model.AppError)
 	CopyFileInfos(userID string, fileIDs []string) ([]string, *model.AppError)
 	CreateChannel(c request.CTX, channel *model.Channel, addMember bool) (*model.Channel, *model.AppError)
 	CreateChannelWithUser(c request.CTX, channel *model.Channel, userID string) (*model.Channel, *model.AppError)
@@ -664,6 +665,7 @@ type AppIface interface {
 	GetGroupMemberUsers(groupID string) ([]*model.User, *model.AppError)
 	GetGroupMemberUsersPage(groupID string, page int, perPage int, viewRestrictions *model.ViewUsersRestrictions) ([]*model.User, int, *model.AppError)
 	GetGroupMemberUsersSortedPage(groupID string, page int, perPage int, viewRestrictions *model.ViewUsersRestrictions, teammateNameDisplay string) ([]*model.User, int, *model.AppError)
+	GetGroupMessageMembersCommonTeams(c request.CTX, channelID string) ([]*model.Team, *model.AppError)
 	GetGroupSyncable(groupID string, syncableID string, syncableType model.GroupSyncableType) (*model.GroupSyncable, *model.AppError)
 	GetGroupSyncables(groupID string, syncableType model.GroupSyncableType) ([]*model.GroupSyncable, *model.AppError)
 	GetGroups(page, perPage int, opts model.GroupSearchOpts, viewRestrictions *model.ViewUsersRestrictions) ([]*model.Group, *model.AppError)

server/channels/app/channel.go

@@ -11,6 +11,8 @@ import (
 	"net/http"
 	"strings"
 
+	"github.com/mattermost/mattermost/server/v8/channels/utils"
+
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/plugin"
 	"github.com/mattermost/mattermost/server/public/shared/i18n"
@@ -3478,6 +3480,231 @@ func (a *App) getDirectChannel(c request.CTX, userID, otherUserID string) (*mode
 	return a.Srv().getDirectChannel(c, userID, otherUserID)
 }
 
+func (a *App) GetGroupMessageMembersCommonTeams(c request.CTX, channelID string) ([]*model.Team, *model.AppError) {
+	channel, appErr := a.GetChannel(c, channelID)
+	if appErr != nil {
+		return nil, appErr
+	}
+
+	if channel.Type != model.ChannelTypeGroup {
+		return nil, model.NewAppError("GetGroupMessageMembersCommonTeams", "app.channel.get_common_teams.incorrect_channel_type", nil, "", http.StatusBadRequest)
+	}
+
+	users, appErr := a.GetUsersInChannel(&model.UserGetOptions{
+		PerPage:     model.ChannelGroupMaxUsers,
+		Page:        0,
+		InChannelId: channelID,
+		Inactive:    false,
+		Active:      true,
+	})
+
+	var userIDs = make([]string, len(users))
+	for i := 0; i < len(users); i++ {
+		userIDs[i] = users[i].Id
+	}
+
+	commonTeamIDs, err := a.Srv().Store().Team().GetCommonTeamIDsForMultipleUsers(userIDs)
+	if err != nil {
+		return nil, model.NewAppError("GetGroupMessageMembersCommonTeams", "app.channel.get_common_teams.store_get_common_teams_error", nil, "", http.StatusInternalServerError).Wrap(err)
+	}
+
+	teams := []*model.Team{}
+	if len(commonTeamIDs) > 0 {
+		teams, appErr = a.GetTeams(commonTeamIDs)
+	}
+
+	return teams, appErr
+}
+
+func (a *App) ConvertGroupMessageToChannel(c request.CTX, convertedByUserId string, gmConversionRequest *model.GroupMessageConversionRequestBody) (*model.Channel, *model.AppError) {
+	originalChannel, appErr := a.GetChannel(c, gmConversionRequest.ChannelID)
+	if appErr != nil {
+		return nil, appErr
+	}
+
+	appErr = a.validateForConvertGroupMessageToChannel(c, convertedByUserId, originalChannel, gmConversionRequest)
+	if appErr != nil {
+		return nil, appErr
+	}
+
+	toUpdate := originalChannel.DeepCopy()
+	toUpdate.Type = model.ChannelTypePrivate
+	toUpdate.TeamId = gmConversionRequest.TeamID
+	toUpdate.Name = gmConversionRequest.Name
+	toUpdate.DisplayName = gmConversionRequest.DisplayName
+
+	updatedChannel, appErr := a.UpdateChannel(c, toUpdate)
+	if appErr != nil {
+		return nil, appErr
+	}
+
+	a.Srv().Platform().InvalidateCacheForChannel(originalChannel)
+
+	users, appErr := a.GetUsersInChannelPage(&model.UserGetOptions{
+		InChannelId: gmConversionRequest.ChannelID,
+		Page:        0,
+		PerPage:     model.ChannelGroupMaxUsers,
+	}, false)
+	if appErr != nil {
+		return nil, appErr
+	}
+
+	_ = a.setSidebarCategoriesForConvertedGroupMessage(c, gmConversionRequest, users)
+	_ = a.postMessageForConvertGroupMessageToChannel(c, gmConversionRequest.ChannelID, convertedByUserId, users)
+	return updatedChannel, nil
+}
+
+func (a *App) setSidebarCategoriesForConvertedGroupMessage(c request.CTX, gmConversionRequest *model.GroupMessageConversionRequestBody, channelUsers []*model.User) *model.AppError {
+	// First we'll delete channel from everyone's sidebar. Only the members of GM
+	// can have it in sidebar, so we can delete the channel from all SidebarChannels entries.
+	err := a.Srv().Store().Channel().DeleteAllSidebarChannelForChannel(gmConversionRequest.ChannelID)
+	if err != nil {
+		return model.NewAppError(
+			"setSidebarCategoriesForConvertedGroupMessage",
+			"app.channel.gm_conversion_set_categories.delete_all.error",
+			nil,
+			"",
+			http.StatusInternalServerError,
+		).Wrap(err)
+	}
+
+	// Now that we've deleted existing entries, we can set the channel in default "Channels" category
+	// for all GM members
+	for _, user := range channelUsers {
+		categories, appErr := a.GetSidebarCategories(c, user.Id, &store.SidebarCategorySearchOpts{
+			TeamID: gmConversionRequest.TeamID,
+			Type:   model.SidebarCategoryChannels,
+		})
+
+		if appErr != nil {
+			mlog.Error("Failed to search sidebar categories for user for adding converted GM")
+			continue
+		}
+
+		if len(categories.Categories) < 1 {
+			// It is normal for user to not have the default category.
+			// The default "Channels" category is created when the user first logs in,
+			// and all their channels are moved to this category at the same time.
+			// So its perfectly okay for this condition to occur.
+			continue
+		}
+
+		// when we fetch the default "Channels" category from store layer,
+		// it auto-fills any channels the user has access to but aren't associated to a category in the database.
+		// So what we do is fetch the category, so we get an auto-filled data,
+		// then call update category to persist the data and send the websocket events.
+		channelsCategory := categories.Categories[0]
+		_, appErr = a.UpdateSidebarCategories(c, user.Id, gmConversionRequest.TeamID, []*model.SidebarCategoryWithChannels{channelsCategory})
+		if appErr != nil {
+			mlog.Error("Failed to add converted GM to default sidebar category for user", mlog.String("user_id", user.Id), mlog.Err(err))
+		}
+	}
+
+	return nil
+}
+
+func (a *App) validateForConvertGroupMessageToChannel(c request.CTX, convertedByUserId string, originalChannel *model.Channel, gmConversionRequest *model.GroupMessageConversionRequestBody) *model.AppError {
+	commonTeams, appErr := a.GetGroupMessageMembersCommonTeams(c, originalChannel.Id)
+	if appErr != nil {
+		return appErr
+	}
+
+	teamFound := false
+	for _, team := range commonTeams {
+		if team.Id == gmConversionRequest.TeamID {
+			teamFound = true
+			break
+		}
+	}
+
+	if !teamFound {
+		return model.NewAppError(
+			"validateForConvertGroupMessageToChannel",
+			"app.channel.group_message_conversion.incorrect_team",
+			nil,
+			"",
+			http.StatusBadRequest,
+		)
+	}
+
+	if originalChannel.Type != model.ChannelTypeGroup {
+		return model.NewAppError(
+			"ConvertGroupMessageToChannel",
+			"app.channel.group_message_conversion.original_channel_not_gm",
+			nil,
+			"",
+			http.StatusNotFound,
+		)
+	}
+
+	channelMember, appErr := a.GetChannelMember(c, gmConversionRequest.ChannelID, convertedByUserId)
+	if appErr != nil {
+		return appErr
+	}
+
+	if channelMember == nil {
+		return model.NewAppError("ConvertGroupMessageToChannel", "app.channel.group_message_conversion.channel_member_missing", nil, "", http.StatusNotFound)
+	}
+
+	// apply dummy changes to check validity
+	clone := originalChannel.DeepCopy()
+	clone.Type = model.ChannelTypePrivate
+	clone.Name = gmConversionRequest.Name
+	clone.DisplayName = gmConversionRequest.DisplayName
+	return clone.IsValid()
+}
+
+func (a *App) postMessageForConvertGroupMessageToChannel(c request.CTX, channelID, convertedByUserId string, channelUsers []*model.User) *model.AppError {
+	convertedByUser, appErr := a.GetUser(convertedByUserId)
+	if appErr != nil {
+		return appErr
+	}
+
+	userIDs := make([]string, len(channelUsers))
+	usernames := make([]string, len(channelUsers))
+	for i, user := range channelUsers {
+		userIDs[i] = user.Id
+		usernames[i] = user.Username
+	}
+
+	message := i18n.T(
+		"api.channel.group_message.converted.to_private_channel",
+		map[string]any{
+			"ConvertedByUsername": convertedByUser.Username,
+			"GMMembers":           utils.JoinList(usernames),
+		})
+
+	post := &model.Post{
+		ChannelId: channelID,
+		Message:   message,
+		Type:      model.PostTypeGMConvertedToChannel,
+		UserId:    convertedByUserId,
+	}
+
+	// these props are used for re-constructing a localized message on the client
+	post.AddProp("convertedByUserId", convertedByUser.Id)
+	post.AddProp("gmMembersDuringConversionIDs", userIDs)
+
+	channel, appErr := a.GetChannel(c, channelID)
+	if appErr != nil {
+		return appErr
+	}
+
+	if _, appErr := a.CreatePost(c, post, channel, false, true); appErr != nil {
+		mlog.Error("Failed to create post for notifying about GM converted to private channel", mlog.Err(appErr))
+
+		return model.NewAppError(
+			"postMessageForConvertGroupMessageToChannel",
+			"app.channel.group_message_conversion.post_message.error",
+			nil,
+			"",
+			http.StatusInternalServerError,
+		).Wrap(appErr)
+	}
+
+	return nil
+}
+
 func (s *Server) getDirectChannel(c request.CTX, userID, otherUserID string) (*model.Channel, *model.AppError) {
 	channel, nErr := s.Store().Channel().GetByName("", model.GetDMNameFromIds(userID, otherUserID), true)
 	if nErr != nil {

server/channels/app/channel_test.go

@@ -11,6 +11,13 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
+
+	"github.com/mattermost/mattermost/server/v8/channels/store"
+
+	"github.com/mattermost/mattermost/server/v8/channels/app/teams"
+	"github.com/mattermost/mattermost/server/v8/channels/app/users"
+	"github.com/mattermost/mattermost/server/v8/channels/store/sqlstore"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
@@ -2450,3 +2457,210 @@ func TestIsCRTEnabledForUser(t *testing.T) {
 		})
 	}
 }
+
+func TestGetGroupMessageMembersCommonTeams(t *testing.T) {
+	th := SetupWithStoreMock(t)
+	defer th.TearDown()
+
+	mockStore := th.App.Srv().Store().(*mocks.Store)
+
+	mockChannelStore := mocks.ChannelStore{}
+	mockStore.On("Channel").Return(&mockChannelStore)
+	mockChannelStore.On("Get", "gm_channel_id", true).Return(&model.Channel{Type: model.ChannelTypeGroup}, nil)
+
+	mockTeamStore := mocks.TeamStore{}
+	mockStore.On("Team").Return(&mockTeamStore)
+
+	th.App.Srv().Store().Team()
+
+	mockTeamStore.On("GetCommonTeamIDsForMultipleUsers", []string{"user_id_1", "user_id_2"}).Return([]string{"team_id_1", "team_id_2", "team_id_3"}, nil).Times(1)
+	mockTeamStore.On("GetMany", []string{"team_id_1", "team_id_2", "team_id_3"}).Return(
+		[]*model.Team{
+			{DisplayName: "Team 1"},
+			{DisplayName: "Team 2"},
+			{DisplayName: "Team 3"},
+		},
+		nil,
+	)
+
+	mockUserStore := mocks.UserStore{}
+	mockStore.On("User").Return(&mockUserStore)
+	options := &model.UserGetOptions{
+		PerPage:     model.ChannelGroupMaxUsers,
+		Page:        0,
+		InChannelId: "gm_channel_id",
+		Inactive:    false,
+		Active:      true,
+	}
+	mockUserStore.On("GetProfilesInChannel", options).Return([]*model.User{
+		{
+			Id: "user_id_1",
+		},
+		{
+			Id: "user_id_2",
+		},
+	}, nil)
+
+	var err error
+	th.App.ch.srv.teamService, err = teams.New(teams.ServiceConfig{
+		TeamStore:    &mockTeamStore,
+		ChannelStore: &mockChannelStore,
+		GroupStore:   &mocks.GroupStore{},
+		Users:        th.App.ch.srv.userService,
+		WebHub:       th.App.ch.srv.platform,
+		ConfigFn:     th.App.ch.srv.platform.Config,
+		LicenseFn:    th.App.ch.srv.License,
+	})
+	require.NoError(t, err)
+
+	commonTeams, appErr := th.App.GetGroupMessageMembersCommonTeams(th.Context, "gm_channel_id")
+	require.Nil(t, appErr)
+	require.Equal(t, 3, len(commonTeams))
+
+	// case of no common teams
+	mockTeamStore.On("GetCommonTeamIDsForMultipleUsers", []string{"user_id_1", "user_id_2"}).Return([]string{}, nil)
+	commonTeams, appErr = th.App.GetGroupMessageMembersCommonTeams(th.Context, "gm_channel_id")
+	require.Nil(t, appErr)
+	require.Equal(t, 0, len(commonTeams))
+}
+
+func TestConvertGroupMessageToChannel(t *testing.T) {
+	th := SetupWithStoreMock(t)
+	defer th.TearDown()
+
+	mockStore := th.App.Srv().Store().(*mocks.Store)
+
+	mockChannelStore := mocks.ChannelStore{}
+	mockStore.On("Channel").Return(&mockChannelStore)
+	mockChannelStore.On("Get", "channelidchannelidchanneli", true).Return(&model.Channel{
+		Id:       "channelidchannelidchanneli",
+		CreateAt: time.Now().Unix(),
+		UpdateAt: time.Now().Unix(),
+		Type:     model.ChannelTypeGroup,
+	}, nil)
+	mockChannelStore.On("Update", mock.AnythingOfType("*model.Channel")).Return(&model.Channel{}, nil)
+	mockChannelStore.On("InvalidateChannel", "channelidchannelidchanneli")
+	mockChannelStore.On("InvalidateChannelByName", "team_id_1", "new_name").Times(1)
+	mockChannelStore.On("InvalidateChannelByName", "dm", "")
+	mockChannelStore.On("GetMember", sqlstore.WithMaster(context.Background()), "channelidchannelidchanneli", "user_id_1").Return(&model.ChannelMember{}, nil).Times(1)
+	mockChannelStore.On("GetMember", context.Background(), "channelidchannelidchanneli", "user_id_1").Return(&model.ChannelMember{}, nil).Times(1)
+	mockChannelStore.On("InvalidatePinnedPostCount", "channelidchannelidchanneli")
+	mockChannelStore.On("GetAllChannelMembersNotifyPropsForChannel", "channelidchannelidchanneli", true).Return(map[string]model.StringMap{}, nil)
+	mockChannelStore.On("IncrementMentionCount", "", []string{}, true, false).Return(nil)
+	mockChannelStore.On("DeleteAllSidebarChannelForChannel", "channelidchannelidchanneli").Return(nil)
+	mockChannelStore.On("GetSidebarCategories", "user_id_1", &store.SidebarCategorySearchOpts{TeamID: "team_id_1", ExcludeTeam: false, Type: "channels"}).Return(
+		&model.OrderedSidebarCategories{
+			Categories: model.SidebarCategoriesWithChannels{
+				{
+					SidebarCategory: model.SidebarCategory{
+						Type: model.SidebarCategoryChannels,
+					},
+				},
+			},
+		}, nil)
+	mockChannelStore.On("GetSidebarCategories", "user_id_2", &store.SidebarCategorySearchOpts{TeamID: "team_id_1", ExcludeTeam: false, Type: "channels"}).Return(
+		&model.OrderedSidebarCategories{
+			Categories: model.SidebarCategoriesWithChannels{
+				{
+					SidebarCategory: model.SidebarCategory{
+						Type: model.SidebarCategoryChannels,
+					},
+				},
+			},
+		}, nil)
+	mockChannelStore.On("UpdateSidebarCategories", "user_id_1", "team_id_1", mock.Anything).Return(
+		[]*model.SidebarCategoryWithChannels{
+			{
+				SidebarCategory: model.SidebarCategory{
+					Type: model.SidebarCategoryChannels,
+				},
+			},
+		},
+		[]*model.SidebarCategoryWithChannels{
+			{
+				SidebarCategory: model.SidebarCategory{
+					Type: model.SidebarCategoryChannels,
+				},
+			},
+		},
+		nil,
+	)
+	mockChannelStore.On("UpdateSidebarCategories", "user_id_2", "team_id_1", mock.Anything).Return(
+		[]*model.SidebarCategoryWithChannels{
+			{
+				SidebarCategory: model.SidebarCategory{
+					Type: model.SidebarCategoryChannels,
+				},
+			},
+		},
+		[]*model.SidebarCategoryWithChannels{
+			{
+				SidebarCategory: model.SidebarCategory{
+					Type: model.SidebarCategoryChannels,
+				},
+			},
+		},
+		nil,
+	)
+
+	mockTeamStore := mocks.TeamStore{}
+	mockStore.On("Team").Return(&mockTeamStore)
+	mockTeamStore.On("GetMember", sqlstore.WithMaster(context.Background()), "team_id_1", "user_id_1").Return(&model.TeamMember{}, nil)
+	mockTeamStore.On("GetCommonTeamIDsForMultipleUsers", []string{"user_id_1", "user_id_2"}).Return([]string{"team_id_1", "team_id_2", "team_id_3"}, nil).Times(1)
+	mockTeamStore.On("GetMany", []string{"team_id_1", "team_id_2", "team_id_3"}).Return(
+		[]*model.Team{
+			{Id: "team_id_1", DisplayName: "Team 1"},
+			{Id: "team_id_2", DisplayName: "Team 2"},
+			{Id: "team_id_3", DisplayName: "Team 3"},
+		},
+		nil,
+	)
+
+	mockUserStore := mocks.UserStore{}
+	mockStore.On("User").Return(&mockUserStore)
+	mockUserStore.On("Get", context.Background(), "user_id_1").Return(&model.User{Username: "username_1"}, nil)
+	mockUserStore.On("GetProfilesInChannel", mock.AnythingOfType("*model.UserGetOptions")).Return([]*model.User{
+		{Id: "user_id_1", Username: "user_id_1"},
+		{Id: "user_id_2", Username: "user_id_2"},
+	}, nil)
+	mockUserStore.On("GetAllProfilesInChannel", mock.Anything, mock.Anything, mock.Anything).Return(map[string]*model.User{}, nil)
+
+	mockPostStore := mocks.PostStore{}
+	mockStore.On("Post").Return(&mockPostStore)
+	mockPostStore.On("Save", mock.AnythingOfType("*model.Post")).Return(&model.Post{}, nil)
+	mockPostStore.On("InvalidateLastPostTimeCache", "channelidchannelidchanneli")
+
+	var err error
+
+	th.App.ch.srv.userService, err = users.New(users.ServiceConfig{
+		UserStore:    &mockUserStore,
+		ConfigFn:     th.App.ch.srv.platform.Config,
+		SessionStore: &mocks.SessionStore{},
+		OAuthStore:   &mocks.OAuthStore{},
+		LicenseFn:    th.App.ch.srv.License,
+	})
+	require.NoError(t, err)
+
+	th.App.ch.srv.teamService, err = teams.New(teams.ServiceConfig{
+		TeamStore:    &mockTeamStore,
+		ChannelStore: &mockChannelStore,
+		GroupStore:   &mocks.GroupStore{},
+		Users:        th.App.ch.srv.userService,
+		WebHub:       th.App.ch.srv.platform,
+		ConfigFn:     th.App.ch.srv.platform.Config,
+		LicenseFn:    th.App.ch.srv.License,
+	})
+	require.NoError(t, err)
+
+	conversionRequest := &model.GroupMessageConversionRequestBody{
+		ChannelID:   "channelidchannelidchanneli",
+		TeamID:      "team_id_1",
+		Name:        "new_name",
+		DisplayName: "New Display Name",
+	}
+
+	convertedChannel, appErr := th.App.ConvertGroupMessageToChannel(th.Context, "user_id_1", conversionRequest)
+	require.Nil(t, appErr)
+	require.Equal(t, model.ChannelTypePrivate, convertedChannel.Type)
+
+}

server/channels/app/opentracing/opentracing_layer.go

@@ -1819,6 +1819,28 @@ func (a *OpenTracingAppLayer) ConvertBotToUser(c request.CTX, bot *model.Bot, us
 	return resultVar0, resultVar1
 }
 
+func (a *OpenTracingAppLayer) ConvertGroupMessageToChannel(c request.CTX, convertedByUserId string, gmConversionRequest *model.GroupMessageConversionRequestBody) (*model.Channel, *model.AppError) {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.ConvertGroupMessageToChannel")
+
+	a.ctx = newCtx
+	a.app.Srv().Store().SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store().SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0, resultVar1 := a.app.ConvertGroupMessageToChannel(c, convertedByUserId, gmConversionRequest)
+
+	if resultVar1 != nil {
+		span.LogFields(spanlog.Error(resultVar1))
+		ext.Error.Set(span, true)
+	}
+
+	return resultVar0, resultVar1
+}
+
 func (a *OpenTracingAppLayer) ConvertUserToBot(user *model.User) (*model.Bot, *model.AppError) {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.ConvertUserToBot")
@@ -6666,6 +6688,28 @@ func (a *OpenTracingAppLayer) GetGroupMemberUsersSortedPage(groupID string, page
 	return resultVar0, resultVar1, resultVar2
 }
 
+func (a *OpenTracingAppLayer) GetGroupMessageMembersCommonTeams(c request.CTX, channelID string) ([]*model.Team, *model.AppError) {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.GetGroupMessageMembersCommonTeams")
+
+	a.ctx = newCtx
+	a.app.Srv().Store().SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store().SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0, resultVar1 := a.app.GetGroupMessageMembersCommonTeams(c, channelID)
+
+	if resultVar1 != nil {
+		span.LogFields(spanlog.Error(resultVar1))
+		ext.Error.Set(span, true)
+	}
+
+	return resultVar0, resultVar1
+}
+
 func (a *OpenTracingAppLayer) GetGroupSyncable(groupID string, syncableID string, syncableType model.GroupSyncableType) (*model.GroupSyncable, *model.AppError) {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.GetGroupSyncable")

server/channels/store/opentracinglayer/opentracinglayer.go

@@ -865,6 +865,24 @@ func (s *OpenTracingLayerChannelStore) Delete(channelID string, timestamp int64)
 	return err
 }
 
+func (s *OpenTracingLayerChannelStore) DeleteAllSidebarChannelForChannel(channelID string) error {
+	origCtx := s.Root.Store.Context()
+	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "ChannelStore.DeleteAllSidebarChannelForChannel")
+	s.Root.Store.SetContext(newCtx)
+	defer func() {
+		s.Root.Store.SetContext(origCtx)
+	}()
+
+	defer span.Finish()
+	err := s.ChannelStore.DeleteAllSidebarChannelForChannel(channelID)
+	if err != nil {
+		span.LogFields(spanlog.Error(err))
+		ext.Error.Set(span, true)
+	}
+
+	return err
+}
+
 func (s *OpenTracingLayerChannelStore) DeleteSidebarCategory(categoryID string) error {
 	origCtx := s.Root.Store.Context()
 	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "ChannelStore.DeleteSidebarCategory")
@@ -9554,6 +9572,24 @@ func (s *OpenTracingLayerTeamStore) GetChannelUnreadsForTeam(teamID string, user
 	return result, err
 }
 
+func (s *OpenTracingLayerTeamStore) GetCommonTeamIDsForMultipleUsers(userIDs []string) ([]string, error) {
+	origCtx := s.Root.Store.Context()
+	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "TeamStore.GetCommonTeamIDsForMultipleUsers")
+	s.Root.Store.SetContext(newCtx)
+	defer func() {
+		s.Root.Store.SetContext(origCtx)
+	}()
+
+	defer span.Finish()
+	result, err := s.TeamStore.GetCommonTeamIDsForMultipleUsers(userIDs)
+	if err != nil {
+		span.LogFields(spanlog.Error(err))
+		ext.Error.Set(span, true)
+	}
+
+	return result, err
+}
+
 func (s *OpenTracingLayerTeamStore) GetCommonTeamIDsForTwoUsers(userID string, otherUserID string) ([]string, error) {
 	origCtx := s.Root.Store.Context()
 	span, newCtx := tracing.StartSpanWithParentByContext(s.Root.Store.Context(), "TeamStore.GetCommonTeamIDsForTwoUsers")

server/channels/store/retrylayer/retrylayer.go

@@ -934,6 +934,27 @@ func (s *RetryLayerChannelStore) Delete(channelID string, timestamp int64) error
 
 }
 
+func (s *RetryLayerChannelStore) DeleteAllSidebarChannelForChannel(channelID string) error {
+
+	tries := 0
+	for {
+		err := s.ChannelStore.DeleteAllSidebarChannelForChannel(channelID)
+		if err == nil {
+			return nil
+		}
+		if !isRepeatableError(err) {
+			return err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
 func (s *RetryLayerChannelStore) DeleteSidebarCategory(categoryID string) error {
 
 	tries := 0
@@ -10906,6 +10927,27 @@ func (s *RetryLayerTeamStore) GetChannelUnreadsForTeam(teamID string, userID str
 
 }
 
+func (s *RetryLayerTeamStore) GetCommonTeamIDsForMultipleUsers(userIDs []string) ([]string, error) {
+
+	tries := 0
+	for {
+		result, err := s.TeamStore.GetCommonTeamIDsForMultipleUsers(userIDs)
+		if err == nil {
+			return result, nil
+		}
+		if !isRepeatableError(err) {
+			return result, err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return result, err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
 func (s *RetryLayerTeamStore) GetCommonTeamIDsForTwoUsers(userID string, otherUserID string) ([]string, error) {
 
 	tries := 0

server/channels/store/sqlstore/channel_store_categories.go

@@ -553,6 +553,10 @@ func (s SqlChannelStore) getSidebarCategoriesT(db dbSelecter, userId string, opt
 		query = query.Where(sq.Eq{"SidebarCategories.TeamId": opts.TeamID})
 	}
 
+	if opts.Type != "" {
+		query = query.Where(sq.Eq{"SidebarCategories.Type": opts.Type})
+	}
+
 	sql, args, err := query.ToSql()
 	if err != nil {
 		return nil, errors.Wrap(err, "sidebar_categories_tosql")
@@ -1125,3 +1129,18 @@ func (s SqlChannelStore) DeleteSidebarCategory(categoryId string) (err error) {
 
 	return nil
 }
+
+func (s SqlChannelStore) DeleteAllSidebarChannelForChannel(channelID string) error {
+	query, args, err := s.getQueryBuilder().
+		Delete("SidebarChannels").
+		Where(sq.Eq{
+			"ChannelId": channelID,
+		}).ToSql()
+
+	if err != nil {
+		return errors.Wrap(err, "delete_all_sidebar_channel_for_channel_to_sql")
+	}
+
+	_, err = s.GetMasterX().Exec(query, args...)
+	return err
+}

server/channels/store/sqlstore/team_store.go

@@ -1513,6 +1513,46 @@ func (s SqlTeamStore) GetCommonTeamIDsForTwoUsers(userID, otherUserID string) ([
 	return teamIDs, nil
 }
 
+func (s SqlTeamStore) GetCommonTeamIDsForMultipleUsers(userIDs []string) ([]string, error) {
+	subQuery := s.getSubQueryBuilder().
+		Select("TeamId, UserId").
+		From("TeamMembers").
+		Where(sq.Eq{
+			"UserId":   userIDs,
+			"DeleteAt": 0,
+		})
+
+	subQuerySQL, subQueryParams, err := subQuery.ToSql()
+	if err != nil {
+		return nil, errors.Wrap(err, "GetCommonTeamIDsForMultipleUsers_subquery_toSQL")
+	}
+
+	query := s.getQueryBuilder().
+		Select("t.Id").
+		From("Teams AS t").
+		Join("("+subQuerySQL+") AS tm ON t.Id = tm.TeamId", subQueryParams...).
+		Where(sq.Eq{
+			"t.DeleteAt": 0,
+		}).
+		GroupBy("t.Id").
+		Having(sq.Eq{
+			"COUNT(UserId)": len(userIDs),
+		})
+
+	querySQL, args, err := query.ToSql()
+	if err != nil {
+		return nil, errors.Wrap(err, "GetCommonTeamIDsForMultipleUsers_query_toSQL")
+	}
+
+	var teamIDs []string
+
+	if err := s.GetReplicaX().Select(&teamIDs, querySQL, args...); err != nil {
+		return nil, errors.Wrapf(err, "failed to find common team for users %v", userIDs)
+	}
+
+	return teamIDs, nil
+}
+
 // GetTeamMembersForExport gets the various teams for which a user, denoted by userId, is a part of.
 func (s SqlTeamStore) GetTeamMembersForExport(userId string) ([]*model.TeamMemberForExport, error) {
 	members := []*model.TeamMemberForExport{}

server/channels/store/store.go

@@ -174,6 +174,8 @@ type TeamStore interface {
 	// GetCommonTeamIDsForTwoUsers returns the intersection of all the teams to which the specified
 	// users belong.
 	GetCommonTeamIDsForTwoUsers(userID, otherUserID string) ([]string, error)
+
+	GetCommonTeamIDsForMultipleUsers(userIDs []string) ([]string, error)
 }
 
 type ChannelStore interface {
@@ -287,6 +289,7 @@ type ChannelStore interface {
 	UpdateSidebarChannelsByPreferences(preferences model.Preferences) error
 	DeleteSidebarChannelsByPreferences(preferences model.Preferences) error
 	DeleteSidebarCategory(categoryID string) error
+	DeleteAllSidebarChannelForChannel(channelID string) error
 	GetAllChannelsForExportAfter(limit int, afterID string) ([]*model.ChannelForExport, error)
 	GetAllDirectChannelsForExportAfter(limit int, afterID string) ([]*model.DirectChannelForExport, error)
 	GetChannelMembersForExport(userID string, teamID string, includeArchivedChannel bool) ([]*model.ChannelMemberForExport, error)
@@ -1100,6 +1103,7 @@ type PostReminderMetadata struct {
 type SidebarCategorySearchOpts struct {
 	TeamID      string
 	ExcludeTeam bool
+	Type        model.SidebarCategoryType
 }
 
 // Ensure store service adapter implements `product.StoreService`

server/channels/store/storetest/mocks/ChannelStore.go

@@ -336,6 +336,20 @@ func (_m *ChannelStore) Delete(channelID string, timestamp int64) error {
 	return r0
 }
 
+// DeleteAllSidebarChannelForChannel provides a mock function with given fields: channelID
+func (_m *ChannelStore) DeleteAllSidebarChannelForChannel(channelID string) error {
+	ret := _m.Called(channelID)
+
+	var r0 error
+	if rf, ok := ret.Get(0).(func(string) error); ok {
+		r0 = rf(channelID)
+	} else {
+		r0 = ret.Error(0)
+	}
+
+	return r0
+}
+
 // DeleteSidebarCategory provides a mock function with given fields: categoryID
 func (_m *ChannelStore) DeleteSidebarCategory(categoryID string) error {
 	ret := _m.Called(categoryID)

server/channels/store/storetest/mocks/TeamStore.go

@@ -419,6 +419,32 @@ func (_m *TeamStore) GetChannelUnreadsForTeam(teamID string, userID string) ([]*
 	return r0, r1
 }
 
+// GetCommonTeamIDsForMultipleUsers provides a mock function with given fields: userIDs
+func (_m *TeamStore) GetCommonTeamIDsForMultipleUsers(userIDs []string) ([]string, error) {
+	ret := _m.Called(userIDs)
+
+	var r0 []string
+	var r1 error
+	if rf, ok := ret.Get(0).(func([]string) ([]string, error)); ok {
+		return rf(userIDs)
+	}
+	if rf, ok := ret.Get(0).(func([]string) []string); ok {
+		r0 = rf(userIDs)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).([]string)
+		}
+	}
+
+	if rf, ok := ret.Get(1).(func([]string) error); ok {
+		r1 = rf(userIDs)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
 // GetCommonTeamIDsForTwoUsers provides a mock function with given fields: userID, otherUserID
 func (_m *TeamStore) GetCommonTeamIDsForTwoUsers(userID string, otherUserID string) ([]string, error) {
 	ret := _m.Called(userID, otherUserID)

server/channels/store/storetest/team_store.go

@@ -73,6 +73,7 @@ func TestTeamStore(t *testing.T, ss store.Store) {
 	t.Run("GetTeamMembersForExport", func(t *testing.T) { testTeamStoreGetTeamMembersForExport(t, ss) })
 	t.Run("GetTeamsForUserWithPagination", func(t *testing.T) { testTeamMembersWithPagination(t, ss) })
 	t.Run("GroupSyncedTeamCount", func(t *testing.T) { testGroupSyncedTeamCount(t, ss) })
+	t.Run("GetCommonTeamIDsForMultipleUsers", func(t *testing.T) { testGetCommonTeamIDsForMultipleUsers(t, ss) })
 }
 
 func testTeamStoreSave(t *testing.T, ss store.Store) {
@@ -3618,3 +3619,203 @@ func testGroupSyncedTeamCount(t *testing.T, ss store.Store) {
 	require.NoError(t, err)
 	require.GreaterOrEqual(t, countAfter, count+1)
 }
+
+func testGetCommonTeamIDsForMultipleUsers(t *testing.T, ss store.Store) {
+	// Creating teams
+
+	// Team 1
+	t1 := model.Team{}
+	t1.DisplayName = "Team 1"
+	t1.Name = NewTestId()
+	t1.Email = MakeEmail()
+	t1.Type = model.TeamOpen
+	_, err := ss.Team().Save(&t1)
+	require.NoError(t, err)
+
+	// Team 2
+	t2 := model.Team{}
+	t2.DisplayName = "Team 2"
+	t2.Name = NewTestId()
+	t2.Email = MakeEmail()
+	t2.Type = model.TeamOpen
+	_, err = ss.Team().Save(&t2)
+	require.NoError(t, err)
+
+	// Team 3
+	t3 := model.Team{}
+	t3.DisplayName = "Team 3"
+	t3.Name = NewTestId()
+	t3.Email = MakeEmail()
+	t3.Type = model.TeamOpen
+	_, err = ss.Team().Save(&t3)
+	require.NoError(t, err)
+
+	// Creating users
+
+	// User 1
+	u1 := model.User{}
+	u1.Email = MakeEmail()
+	u1.Nickname = NewTestId()
+	_, err = ss.User().Save(&u1)
+	require.NoError(t, err)
+
+	// User 2
+	u2 := model.User{}
+	u2.Email = MakeEmail()
+	u2.Nickname = NewTestId()
+	_, err = ss.User().Save(&u2)
+	require.NoError(t, err)
+
+	t.Run("multiple common teams exist", func(t *testing.T) {
+		// Add user 1 in team 1 and 2
+		m1 := &model.TeamMember{TeamId: t1.Id, UserId: u1.Id}
+		m2 := &model.TeamMember{TeamId: t2.Id, UserId: u1.Id}
+
+		// Add user 2 in team1, 2 and 3
+		m3 := &model.TeamMember{TeamId: t1.Id, UserId: u2.Id}
+		m4 := &model.TeamMember{TeamId: t2.Id, UserId: u2.Id}
+		m5 := &model.TeamMember{TeamId: t3.Id, UserId: u2.Id}
+
+		// Save team memberships
+		_, nErr := ss.Team().SaveMultipleMembers([]*model.TeamMember{m1, m2, m3, m4, m5}, -1)
+		require.NoError(t, nErr)
+
+		// Find common teams between user 1 and user 2
+		commonTeamIDs, err2 := ss.Team().GetCommonTeamIDsForMultipleUsers([]string{u1.Id, u2.Id})
+		require.NoError(t, err2)
+		require.Equal(t, 2, len(commonTeamIDs))
+		require.Contains(t, commonTeamIDs, t1.Id)
+		require.Contains(t, commonTeamIDs, t2.Id)
+
+		// cleanup
+		err2 = ss.Team().RemoveAllMembersByUser(u1.Id)
+		require.NoError(t, err2)
+
+		err2 = ss.Team().RemoveAllMembersByUser(u2.Id)
+		require.NoError(t, err2)
+	})
+
+	t.Run("single common teams exist", func(t *testing.T) {
+		// Add user 1 in team 1 and 2
+		m1 := &model.TeamMember{TeamId: t1.Id, UserId: u1.Id}
+		m2 := &model.TeamMember{TeamId: t2.Id, UserId: u1.Id}
+
+		// Add user 2 in team1, 2 and 3
+		m3 := &model.TeamMember{TeamId: t1.Id, UserId: u2.Id}
+		m5 := &model.TeamMember{TeamId: t3.Id, UserId: u2.Id}
+
+		// Save team memberships
+		_, nErr := ss.Team().SaveMultipleMembers([]*model.TeamMember{m1, m2, m3, m5}, -1)
+		require.NoError(t, nErr)
+
+		// Find common teams between user 1 and user 2
+		commonTeamIDs, err2 := ss.Team().GetCommonTeamIDsForMultipleUsers([]string{u1.Id, u2.Id})
+		require.NoError(t, err2)
+		require.Equal(t, 1, len(commonTeamIDs))
+		require.Contains(t, commonTeamIDs, t1.Id)
+
+		// cleanup
+		err2 = ss.Team().RemoveAllMembersByUser(u1.Id)
+		require.NoError(t, err2)
+
+		err2 = ss.Team().RemoveAllMembersByUser(u2.Id)
+		require.NoError(t, err2)
+	})
+
+	t.Run("no common teams exist", func(t *testing.T) {
+		// Add user 1 in team 1 and 2
+		m1 := &model.TeamMember{TeamId: t1.Id, UserId: u1.Id}
+
+		// Add user 2 in team1, 2 and 3
+		m4 := &model.TeamMember{TeamId: t2.Id, UserId: u2.Id}
+		m5 := &model.TeamMember{TeamId: t3.Id, UserId: u2.Id}
+
+		// Save team memberships
+		_, nErr := ss.Team().SaveMultipleMembers([]*model.TeamMember{m1, m4, m5}, -1)
+		require.NoError(t, nErr)
+
+		// Find common teams between user 1 and user 2
+		commonTeamIDs, err2 := ss.Team().GetCommonTeamIDsForMultipleUsers([]string{u1.Id, u2.Id})
+		require.NoError(t, err2)
+		require.Equal(t, 0, len(commonTeamIDs))
+
+		// cleanup
+		err2 = ss.Team().RemoveAllMembersByUser(u1.Id)
+		require.NoError(t, err2)
+
+		err2 = ss.Team().RemoveAllMembersByUser(u2.Id)
+		require.NoError(t, err2)
+	})
+
+	t.Run("some user have no team members", func(t *testing.T) {
+		// Add user 1 in team 1 and 2
+		m1 := &model.TeamMember{TeamId: t1.Id, UserId: u1.Id}
+		m2 := &model.TeamMember{TeamId: t2.Id, UserId: u1.Id}
+
+		// We'll leave user 2 without any team
+
+		// Save team memberships
+		_, nErr := ss.Team().SaveMultipleMembers([]*model.TeamMember{m1, m2}, -1)
+		require.NoError(t, nErr)
+
+		// Find common teams between user 1 and user 2
+		commonTeamIDs, err2 := ss.Team().GetCommonTeamIDsForMultipleUsers([]string{u1.Id, u2.Id})
+		require.NoError(t, err2)
+		require.Equal(t, 0, len(commonTeamIDs))
+
+		// cleanup
+		err2 = ss.Team().RemoveAllMembersByUser(u1.Id)
+		require.NoError(t, err2)
+	})
+
+	t.Run("more than two users, common teams exist", func(t *testing.T) {
+		// User 3
+		u3 := model.User{}
+		u3.Email = MakeEmail()
+		u3.Nickname = NewTestId()
+		_, err = ss.User().Save(&u3)
+		require.NoError(t, err)
+
+		// User 4
+		u4 := model.User{}
+		u4.Email = MakeEmail()
+		u4.Nickname = NewTestId()
+		_, err = ss.User().Save(&u4)
+		require.NoError(t, err)
+
+		// Add user 1 in team 1 and 2
+		m1 := &model.TeamMember{TeamId: t1.Id, UserId: u1.Id}
+		m2 := &model.TeamMember{TeamId: t2.Id, UserId: u1.Id}
+
+		// Add user 2 in team1, 2 and 3
+		m3 := &model.TeamMember{TeamId: t1.Id, UserId: u2.Id}
+		m4 := &model.TeamMember{TeamId: t2.Id, UserId: u2.Id}
+		m5 := &model.TeamMember{TeamId: t3.Id, UserId: u2.Id}
+
+		// Add user 3 in team 1 and 2
+		m6 := &model.TeamMember{TeamId: t1.Id, UserId: u3.Id}
+		m7 := &model.TeamMember{TeamId: t2.Id, UserId: u3.Id}
+
+		// Add user 4 in team 1 and 2
+		m8 := &model.TeamMember{TeamId: t1.Id, UserId: u4.Id}
+		m9 := &model.TeamMember{TeamId: t2.Id, UserId: u4.Id}
+
+		// Save team memberships
+		_, nErr := ss.Team().SaveMultipleMembers([]*model.TeamMember{m1, m2, m3, m4, m5, m6, m7, m8, m9}, -1)
+		require.NoError(t, nErr)
+
+		// Find common teams between user 1 and user 2
+		commonTeamIDs, err := ss.Team().GetCommonTeamIDsForMultipleUsers([]string{u1.Id, u2.Id, u3.Id, u4.Id})
+		require.NoError(t, err)
+		require.Equal(t, 2, len(commonTeamIDs))
+		require.Contains(t, commonTeamIDs, t1.Id)
+		require.Contains(t, commonTeamIDs, t2.Id)
+
+		// cleanup
+		err = ss.Team().RemoveAllMembersByUser(u1.Id)
+		require.NoError(t, err)
+
+		err = ss.Team().RemoveAllMembersByUser(u2.Id)
+		require.NoError(t, err)
+	})
+}

server/channels/store/timerlayer/timerlayer.go

@@ -827,6 +827,22 @@ func (s *TimerLayerChannelStore) Delete(channelID string, timestamp int64) error
 	return err
 }
 
+func (s *TimerLayerChannelStore) DeleteAllSidebarChannelForChannel(channelID string) error {
+	start := time.Now()
+
+	err := s.ChannelStore.DeleteAllSidebarChannelForChannel(channelID)
+
+	elapsed := float64(time.Since(start)) / float64(time.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ChannelStore.DeleteAllSidebarChannelForChannel", success, elapsed)
+	}
+	return err
+}
+
 func (s *TimerLayerChannelStore) DeleteSidebarCategory(categoryID string) error {
 	start := time.Now()
 
@@ -8606,6 +8622,22 @@ func (s *TimerLayerTeamStore) GetChannelUnreadsForTeam(teamID string, userID str
 	return result, err
 }
 
+func (s *TimerLayerTeamStore) GetCommonTeamIDsForMultipleUsers(userIDs []string) ([]string, error) {
+	start := time.Now()
+
+	result, err := s.TeamStore.GetCommonTeamIDsForMultipleUsers(userIDs)
+
+	elapsed := float64(time.Since(start)) / float64(time.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("TeamStore.GetCommonTeamIDsForMultipleUsers", success, elapsed)
+	}
+	return result, err
+}
+
 func (s *TimerLayerTeamStore) GetCommonTeamIDsForTwoUsers(userID string, otherUserID string) ([]string, error) {
 	start := time.Now()
 

server/channels/utils/humanize.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package utils
+
+import (
+	"strings"
+
+	"github.com/mattermost/mattermost/server/public/shared/i18n"
+)
+
+func JoinList(items []string) string {
+	if len(items) == 0 {
+		return ""
+	} else if len(items) == 1 {
+		return items[0]
+	} else {
+		return i18n.T(
+			"humanize.list_join",
+			map[string]any{
+				"OtherItems": strings.Join(items[:len(items)-1], ", "),
+				"LastItem":   items[len(items)-1],
+			})
+	}
+}

server/i18n/en.json

@@ -295,6 +295,14 @@
     "id": "api.channel.get_channel_moderations.license.error",
     "translation": "Your license does not support channel moderation"
   },
+  {
+    "id": "api.channel.gm_to_channel_conversion.not_allowed_for_user.request_error",
+    "translation": "User is not allowed to convert group message to private channel"
+  },
+  {
+    "id": "api.channel.group_message.converted.to_private_channel",
+    "translation": "{{.ConvertedByUsername}} created this channel from a group message with {{.GMMembers}}."
+  },
   {
     "id": "api.channel.guest_join_channel.post_and_forget",
     "translation": "%v joined the channel as guest."
@@ -4810,6 +4818,14 @@
     "id": "app.channel.get_channels_with_unreads_and_with_mentions.app_error",
     "translation": "Unable to check unreads and mentions"
   },
+  {
+    "id": "app.channel.get_common_teams.incorrect_channel_type",
+    "translation": "Channel is not a group message."
+  },
+  {
+    "id": "app.channel.get_common_teams.store_get_common_teams_error",
+    "translation": "Couldn't fetch common teams."
+  },
   {
     "id": "app.channel.get_deleted.existing.app_error",
     "translation": "Unable to find the existing deleted channel."
@@ -4870,6 +4886,26 @@
     "id": "app.channel.get_unread.app_error",
     "translation": "Unable to get the channel unread messages."
   },
+  {
+    "id": "app.channel.gm_conversion_set_categories.delete_all.error",
+    "translation": "Couldn't delete existing sidebar categories for converted GM."
+  },
+  {
+    "id": "app.channel.group_message_conversion.channel_member_missing",
+    "translation": "Cannot find user's channel membership"
+  },
+  {
+    "id": "app.channel.group_message_conversion.incorrect_team",
+    "translation": "The specified team ID in conversion request does not contain all the group message members"
+  },
+  {
+    "id": "app.channel.group_message_conversion.original_channel_not_gm",
+    "translation": "The channel being converted is not a group message. You can only convert group messages"
+  },
+  {
+    "id": "app.channel.group_message_conversion.post_message.error",
+    "translation": "Failed to create group message to channel conversion post"
+  },
   {
     "id": "app.channel.migrate_channel_members.select.app_error",
     "translation": "Failed to select the batch of channel members."
@@ -8082,6 +8118,10 @@
     "id": "groups.unsupported_syncable_type",
     "translation": "Unsupported syncable type '{{.Value}}'."
   },
+  {
+    "id": "humanize.list_join",
+    "translation": "{{.OtherItems}} and {{.LastItem}}"
+  },
   {
     "id": "import_process.worker.do_job.file_exists",
     "translation": "Unable to process import: file does not exists."

server/public/model/channel.go

@@ -443,3 +443,10 @@ func GetGroupNameFromUserIds(userIds []string) string {
 
 	return hex.EncodeToString(h.Sum(nil))
 }
+
+type GroupMessageConversionRequestBody struct {
+	ChannelID   string `json:"channel_id"`
+	TeamID      string `json:"team_id"`
+	Name        string `json:"name"`
+	DisplayName string `json:"display_name"`
+}

server/public/model/post.go

@@ -45,6 +45,7 @@ const (
 	PostTypeChannelRestored        = "system_channel_restored"
 	PostTypeEphemeral              = "system_ephemeral"
 	PostTypeChangeChannelPrivacy   = "system_change_chan_privacy"
+	PostTypeGMConvertedToChannel   = "system_gm_to_channel"
 	PostTypeAddBotTeamsChannels    = "add_bot_teams_channels"
 	PostTypeSystemWarnMetricStatus = "warn_metric_status"
 	PostTypeMe                     = "me"
@@ -428,7 +429,8 @@ func (o *Post) IsValid(maxPostSize int) *AppError {
 		PostTypeAddBotTeamsChannels,
 		PostTypeSystemWarnMetricStatus,
 		PostTypeReminder,
-		PostTypeMe:
+		PostTypeMe,
+		PostTypeGMConvertedToChannel:
 	default:
 		if !strings.HasPrefix(o.Type, PostCustomTypePrefix) {
 			return NewAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type, http.StatusBadRequest)

webapp/channels/src/actions/team_actions.ts

@@ -7,10 +7,12 @@ import type {UserProfile} from '@mattermost/types/users';
 
 import {TeamTypes} from 'mattermost-redux/action_types';
 import {getChannelStats} from 'mattermost-redux/actions/channels';
+import {logError} from 'mattermost-redux/actions/errors';
 import {savePreferences} from 'mattermost-redux/actions/preferences';
 import * as TeamActions from 'mattermost-redux/actions/teams';
 import {selectTeam} from 'mattermost-redux/actions/teams';
 import {getUser} from 'mattermost-redux/actions/users';
+import {Client4} from 'mattermost-redux/client';
 import {getCurrentChannelId} from 'mattermost-redux/selectors/entities/channels';
 import {getCurrentUserId} from 'mattermost-redux/selectors/entities/users';
 import type {ActionFunc, DispatchFunc, GetStateFunc} from 'mattermost-redux/types/actions';
@@ -114,3 +116,19 @@ export function updateTeamsOrderForUser(teamIds: Array<Team['id']>) {
         dispatch(savePreferences(currentUserId, teamOrderPreferences));
     };
 }
+
+export function getGroupMessageMembersCommonTeams(channelId: string): ActionFunc<Team[], ServerError> {
+    return async (dispatch) => {
+        let teams: Team[];
+
+        try {
+            const response = await Client4.getGroupMessageMembersCommonTeams(channelId);
+            teams = response.data;
+        } catch (error) {
+            dispatch(logError(error as ServerError));
+            return {error: error as ServerError};
+        }
+
+        return {data: teams};
+    };
+}

webapp/channels/src/actions/websocket_actions.jsx

@@ -79,7 +79,14 @@ import {getGroup} from 'mattermost-redux/selectors/entities/groups';
 import {getPost, getMostRecentPostIdInChannel, getTeamIdFromPost} from 'mattermost-redux/selectors/entities/posts';
 import {isCollapsedThreadsEnabled} from 'mattermost-redux/selectors/entities/preferences';
 import {haveISystemPermission, haveITeamPermission} from 'mattermost-redux/selectors/entities/roles';
-import {getMyTeams, getCurrentRelativeTeamUrl, getCurrentTeamId, getCurrentTeamUrl, getTeam} from 'mattermost-redux/selectors/entities/teams';
+import {
+    getMyTeams,
+    getCurrentRelativeTeamUrl,
+    getCurrentTeamId,
+    getCurrentTeamUrl,
+    getTeam,
+    getRelativeTeamUrl,
+} from 'mattermost-redux/selectors/entities/teams';
 import {getNewestThreadInTeam, getThread, getThreads} from 'mattermost-redux/selectors/entities/threads';
 import {getCurrentUser, getCurrentUserId, getUser, getIsManualStatusForUserId, isCurrentUserSystemAdmin} from 'mattermost-redux/selectors/entities/users';
 import {isGuest} from 'mattermost-redux/utils/user_utils';
@@ -619,7 +626,8 @@ export function handleChannelUpdatedEvent(msg) {
 
         const state = doGetState();
         if (channel.id === getCurrentChannelId(state)) {
-            getHistory().replace(`${getCurrentRelativeTeamUrl(state)}/channels/${channel.name}`);
+            // using channel's team_id to ensure we always redirect to current channel even if channel's team changes.
+            getHistory().replace(`${getRelativeTeamUrl(state, channel.team_id)}/channels/${channel.name}`);
         }
     };
 }

webapp/channels/src/actions/websocket_actions.test.jsx

@@ -649,7 +649,10 @@ describe('handleChannelUpdatedEvent', () => {
     test('when a channel is updated', () => {
         const testStore = configureStore(initialState);
 
-        const channel = {id: 'channel'};
+        const channel = {
+            id: 'channel',
+            team_id: 'team',
+        };
         const msg = {data: {channel: JSON.stringify(channel)}};
 
         testStore.dispatch(handleChannelUpdatedEvent(msg));
@@ -662,7 +665,10 @@ describe('handleChannelUpdatedEvent', () => {
     test('should not change URL when current channel is updated', () => {
         const testStore = configureStore(initialState);
 
-        const channel = {id: 'channel'};
+        const channel = {
+            id: 'channel',
+            team_id: 'team',
+        };
         const msg = {data: {channel: JSON.stringify(channel)}};
 
         testStore.dispatch(handleChannelUpdatedEvent(msg));

webapp/channels/src/components/channel_header_dropdown/__snapshots__/channel_header_dropdown.test.tsx.snap

@@ -459,6 +459,43 @@ exports[`components/ChannelHeaderDropdown should match snapshot with no plugin i
       show={false}
       text="Edit Conversation Header"
     />
+    <MenuItemToggleModalRedux
+      dialogProps={
+        Object {
+          "channel": Object {
+            "create_at": 0,
+            "creator_id": "id",
+            "delete_at": 0,
+            "display_name": "name",
+            "group_constrained": false,
+            "header": "header",
+            "id": "test-channel-id",
+            "last_post_at": 0,
+            "last_root_post_at": 0,
+            "name": "DN",
+            "purpose": "purpose",
+            "scheme_id": "id",
+            "team_id": "team_id",
+            "type": "O",
+            "update_at": 0,
+          },
+        }
+      }
+      dialogType={
+        Object {
+          "$$typeof": Symbol(react.memo),
+          "WrappedComponent": [Function],
+          "compare": null,
+          "type": [Function],
+        }
+      }
+      id="convertGMPrivateChannel"
+      modalId="convert_gm_to_channel"
+      show={false}
+      text="Convert to Private Channel"
+    />
+  </MenuGroup>
+  <MenuGroup>
     <Connect(ChannelPermissionGate)
       channelId="test-channel-id"
       permissions={
@@ -1251,6 +1288,43 @@ exports[`components/ChannelHeaderDropdown should match snapshot with plugins 1`]
       show={false}
       text="Edit Conversation Header"
     />
+    <MenuItemToggleModalRedux
+      dialogProps={
+        Object {
+          "channel": Object {
+            "create_at": 0,
+            "creator_id": "id",
+            "delete_at": 0,
+            "display_name": "name",
+            "group_constrained": false,
+            "header": "header",
+            "id": "test-channel-id",
+            "last_post_at": 0,
+            "last_root_post_at": 0,
+            "name": "DN",
+            "purpose": "purpose",
+            "scheme_id": "id",
+            "team_id": "team_id",
+            "type": "O",
+            "update_at": 0,
+          },
+        }
+      }
+      dialogType={
+        Object {
+          "$$typeof": Symbol(react.memo),
+          "WrappedComponent": [Function],
+          "compare": null,
+          "type": [Function],
+        }
+      }
+      id="convertGMPrivateChannel"
+      modalId="convert_gm_to_channel"
+      show={false}
+      text="Convert to Private Channel"
+    />
+  </MenuGroup>
+  <MenuGroup>
     <Connect(ChannelPermissionGate)
       channelId="test-channel-id"
       permissions={

webapp/channels/src/components/channel_header_dropdown/channel_header_dropdown_items.tsx

@@ -15,6 +15,7 @@ import ChannelInviteModal from 'components/channel_invite_modal';
 import ChannelMoveToSubMenuOld from 'components/channel_move_to_sub_menu_old';
 import ChannelNotificationsModal from 'components/channel_notifications_modal';
 import ConvertChannelModal from 'components/convert_channel_modal';
+import ConvertGmToChannelModal from 'components/convert_gm_to_channel_modal';
 import DeleteChannelModal from 'components/delete_channel_modal';
 import EditChannelHeaderModal from 'components/edit_channel_header_modal';
 import EditChannelPurposeModal from 'components/edit_channel_purpose_modal';
@@ -221,6 +222,18 @@ export default class ChannelHeaderDropdown extends React.PureComponent<Props> {
                         dialogProps={{channel}}
                         text={localizeMessage('channel_header.setConversationHeader', 'Edit Conversation Header')}
                     />
+
+                    <Menu.ItemToggleModalRedux
+                        id='convertGMPrivateChannel'
+                        show={channel.type === Constants.GM_CHANNEL && !isArchived && !isReadonly && !isGuest(user.roles)}
+                        modalId={ModalIdentifiers.CONVERT_GM_TO_CHANNEL}
+                        dialogType={ConvertGmToChannelModal}
+                        dialogProps={{channel}}
+                        text={localizeMessage('sidebar_left.sidebar_channel_menu_convert_to_channel', 'Convert to Private Channel')}
+                    />
+                </Menu.Group>
+
+                <Menu.Group divider={divider}>
                     <ChannelPermissionGate
                         channelId={channel.id}
                         teamId={channel.team_id}

webapp/channels/src/components/channel_name_form_field/channel_name_form_field.scss

@@ -0,0 +1,5 @@
+.channel-name-input-field {
+    height: 34px !important;
+    border: 0 !important;
+    border-radius: 0 !important;
+}

webapp/channels/src/components/channel_name_form_field/channel_name_form_field.tsx

@@ -0,0 +1,145 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import React, {useCallback, useEffect, useRef, useState} from 'react';
+import {useIntl} from 'react-intl';
+import {useSelector} from 'react-redux';
+
+import {getCurrentTeam} from 'mattermost-redux/selectors/entities/teams';
+
+import type {CustomMessageInputType} from 'components/widgets/inputs/input/input';
+import Input from 'components/widgets/inputs/input/input';
+import URLInput from 'components/widgets/inputs/url_input/url_input';
+
+import Constants from 'utils/constants';
+import {cleanUpUrlable, getSiteURL, validateChannelUrl} from 'utils/url';
+import {generateSlug, localizeMessage} from 'utils/utils';
+
+export type Props = {
+    value: string;
+    name: string;
+    placeholder: string;
+    onDisplayNameChange: (name: string) => void;
+    onURLChange: (url: string) => void;
+    autoFocus?: boolean;
+    onErrorStateChange?: (isError: boolean) => void;
+}
+
+import './channel_name_form_field.scss';
+
+function validateDisplayName(displayNameParam: string) {
+    const errors: string[] = [];
+
+    const displayName = displayNameParam.trim();
+
+    if (displayName.length < Constants.MIN_CHANNELNAME_LENGTH) {
+        errors.push(localizeMessage('channel_modal.name.longer', 'Channel names must have at least 2 characters.'));
+    }
+
+    if (displayName.length > Constants.MAX_CHANNELNAME_LENGTH) {
+        errors.push(localizeMessage('channel_modal.name.shorter', 'Channel names must have maximum 64 characters.'));
+    }
+
+    return errors;
+}
+
+// Component for input fields for editing channel display name
+// along with stuff to edit its URL.
+const ChannelNameFormField = (props: Props): JSX.Element => {
+    const intl = useIntl();
+    const {formatMessage} = intl;
+
+    const displayNameModified = useRef<boolean>(false);
+    const [displayNameError, setDisplayNameError] = useState<string>('');
+    const displayName = useRef<string>('');
+    const urlModified = useRef<boolean>(false);
+    const [url, setURL] = useState<string>('');
+    const [urlError, setURLError] = useState<string>('');
+    const [inputCustomMessage, setInputCustomMessage] = useState<CustomMessageInputType | null>(null);
+
+    const {name: currentTeamName} = useSelector(getCurrentTeam);
+
+    const handleOnDisplayNameChange = useCallback((e: React.ChangeEvent<HTMLInputElement>) => {
+        e.preventDefault();
+        const {target: {value: updatedDisplayName}} = e;
+
+        const displayNameErrors = validateDisplayName(updatedDisplayName);
+
+        // set error if any, else clear it
+        setDisplayNameError(displayNameErrors.length ? displayNameErrors[displayNameErrors.length - 1] : '');
+        displayName.current = updatedDisplayName;
+        props.onDisplayNameChange(updatedDisplayName);
+
+        if (!urlModified.current) {
+            // if URL isn't explicitly modified, it's derived from the display name
+            const cleanURL = cleanUpUrlable(updatedDisplayName);
+            setURL(cleanURL);
+            setURLError('');
+            props.onURLChange(cleanURL);
+        }
+    }, [props.onDisplayNameChange, props.onURLChange]);
+
+    const handleOnDisplayNameBlur = useCallback(() => {
+        if (displayName.current && !url) {
+            const url = generateSlug();
+            setURL(url);
+            props.onURLChange(url);
+        }
+        if (!displayNameModified.current) {
+            displayNameModified.current = true;
+            setInputCustomMessage(null);
+        }
+    }, [props.onURLChange, displayName.current, url, displayNameModified]);
+
+    const handleOnURLChange = useCallback((e: React.ChangeEvent<HTMLInputElement>) => {
+        e.preventDefault();
+        const {target: {value: url}} = e;
+
+        const cleanURL = url.toLowerCase().replace(/\s/g, '-');
+        const urlErrors = validateChannelUrl(cleanURL, intl) as string[];
+
+        setURLError(urlErrors.length ? urlErrors[urlErrors.length - 1] : '');
+        setURL(cleanURL);
+        urlModified.current = true;
+        props.onURLChange(cleanURL);
+    }, [props.onURLChange]);
+
+    useEffect(() => {
+        if (props.onErrorStateChange) {
+            props.onErrorStateChange(Boolean(displayNameError) || Boolean(urlError));
+        }
+    }, [displayNameError, urlError]);
+
+    return (
+        <React.Fragment>
+            <Input
+                type='text'
+                autoComplete='off'
+                autoFocus={props.autoFocus !== false}
+                required={true}
+                name={props.name}
+                containerClassName={`${props.name}-container`}
+                inputClassName={`${props.name}-input channel-name-input-field`}
+                label={formatMessage({id: 'channel_modal.name.label', defaultMessage: 'Channel name'})}
+                placeholder={props.placeholder}
+                limit={Constants.MAX_CHANNELNAME_LENGTH}
+                value={props.value}
+                customMessage={inputCustomMessage}
+                onChange={handleOnDisplayNameChange}
+                onBlur={handleOnDisplayNameBlur}
+            />
+            <URLInput
+                className='new-channel-modal__url'
+                base={getSiteURL()}
+                path={`${currentTeamName}/channels`}
+                pathInfo={url}
+                limit={Constants.MAX_CHANNELNAME_LENGTH}
+                shortenLength={Constants.DEFAULT_CHANNELURL_SHORTEN_LENGTH}
+                error={urlError}
+                onChange={handleOnURLChange}
+            />
+        </React.Fragment>
+    );
+};
+
+export default ChannelNameFormField;

webapp/channels/src/components/convert_gm_to_channel_modal/convert_gm_to_channel_modal.scss

@@ -0,0 +1,60 @@
+#convert-gm-to-channel-modal {
+    .convert-gm-to-channel-modal-body {
+        display: flex;
+        flex-direction: column;
+        gap: 20px;
+        overflow-y: visible;
+        transition: height 200ms ease;
+
+        &.multi-team {
+            height: 340px;
+        }
+
+        &.single-team {
+            height: 230px;
+        }
+
+        &.loading {
+            height: 152px;
+        }
+
+        &.error {
+            height: unset;
+        }
+
+        .new-channel-modal__url {
+            margin-top: -20px;
+        }
+
+        .loadingIndicator {
+            display: block;
+            width: 32px;
+            height: 32px;
+            margin: 36px auto 48px;
+            font-size: 32px;
+        }
+
+        .conversion-error {
+            color: var(--error-text);
+        }
+    }
+
+    .DropDown__menu {
+        z-index: 999;
+        max-height: 120px;
+    }
+
+    .DropDown__menu-list {
+        z-index: 999;
+        max-height: 120px;
+    }
+
+    .modal-body {
+        overflow: visible;
+    }
+
+    .GenericModal__button.delete.disabled {
+        color: var(--button-color) !important;
+        opacity: 0.65;
+    }
+}

webapp/channels/src/components/convert_gm_to_channel_modal/convert_gm_to_channel_modal.test.tsx

@@ -0,0 +1,177 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import {fireEvent, waitFor} from '@testing-library/react';
+import nock from 'nock';
+import React from 'react';
+import {act} from 'react-dom/test-utils';
+
+import type {Channel} from '@mattermost/types/channels';
+import type {Team} from '@mattermost/types/teams';
+import type {UserProfile} from '@mattermost/types/users';
+import type {DeepPartial} from '@mattermost/types/utilities';
+
+import {Client4} from 'mattermost-redux/client';
+import {Preferences} from 'mattermost-redux/constants';
+
+import ConvertGmToChannelModal from 'components/convert_gm_to_channel_modal/convert_gm_to_channel_modal';
+
+import TestHelper from 'packages/mattermost-redux/test/test_helper';
+import {renderWithFullContext, screen} from 'tests/react_testing_utils';
+
+import type {GlobalState} from 'types/store';
+
+describe('component/ConvertGmToChannelModal', () => {
+    const user1 = TestHelper.fakeUserWithId();
+    const user2 = TestHelper.fakeUserWithId();
+    const user3 = TestHelper.fakeUserWithId();
+
+    const baseProps = {
+        onExited: jest.fn(),
+        channel: {id: 'channel_id_1', type: 'G'} as Channel,
+        actions: {
+            closeModal: jest.fn(),
+            convertGroupMessageToPrivateChannel: jest.fn(),
+            moveChannelsInSidebar: jest.fn(),
+        },
+        profilesInChannel: [user1, user2, user3] as UserProfile[],
+        teammateNameDisplaySetting: Preferences.DISPLAY_PREFER_FULL_NAME,
+        channelsCategoryId: 'sidebar_category_1',
+        currentUserId: user1.id,
+    };
+
+    const baseState: DeepPartial<GlobalState> = {
+        entities: {
+            teams: {
+                teams: {
+                    team_id_1: {id: 'team_id_1', display_name: 'Team 1'} as Team,
+                    team_id_2: {id: 'team_id_2', display_name: 'Team 2'} as Team,
+                },
+                currentTeamId: 'team_id_1',
+            },
+        },
+    };
+
+    test('members part of multiple common teams', async () => {
+        TestHelper.initBasic(Client4);
+        nock(Client4.getBaseRoute()).
+            get('/channels/channel_id_1/common_teams').
+            reply(200, [
+                {id: 'team_id_1', display_name: 'Team 1'},
+                {id: 'team_id_2', display_name: 'Team 2'},
+            ]);
+
+        renderWithFullContext(
+            <ConvertGmToChannelModal {...baseProps}/>,
+            baseState,
+        );
+
+        // we need to use waitFor for first assertion as we have a minimum 1200 ms loading animation in the dialog
+        // before it's content is rendered.
+        await waitFor(
+            () => expect(screen.queryByText('Conversation history will be visible to any channel members')).toBeInTheDocument(),
+            {timeout: 1500},
+        );
+
+        expect(screen.queryByText('Select Team')).toBeInTheDocument();
+        expect(screen.queryByPlaceholderText('Channel name')).toBeInTheDocument();
+        expect(screen.queryByText('Edit')).toBeInTheDocument();
+    });
+
+    test('members part of single common teams', async () => {
+        TestHelper.initBasic(Client4);
+        nock(Client4.getBaseRoute()).
+            get('/channels/channel_id_1/common_teams').
+            reply(200, [
+                {id: 'team_id_1', display_name: 'Team 1'},
+            ]);
+
+        renderWithFullContext(
+            <ConvertGmToChannelModal {...baseProps}/>,
+            baseState,
+        );
+
+        // we need to use waitFor for first assertion as we have a minimum 1200 ms loading animation in the dialog
+        // before it's content is rendered.
+        await waitFor(
+            () => expect(screen.queryByText('Conversation history will be visible to any channel members')).toBeInTheDocument(),
+            {timeout: 1500},
+        );
+
+        expect(screen.queryByText('Select Team')).not.toBeInTheDocument();
+        expect(screen.queryByPlaceholderText('Channel name')).toBeInTheDocument();
+        expect(screen.queryByText('Edit')).toBeInTheDocument();
+    });
+
+    test('members part of no common teams', async () => {
+        TestHelper.initBasic(Client4);
+        nock(Client4.getBaseRoute()).
+            get('/channels/channel_id_1/common_teams').
+            reply(200, []);
+
+        renderWithFullContext(
+            <ConvertGmToChannelModal {...baseProps}/>,
+            baseState,
+        );
+
+        // we need to use waitFor for first assertion as we have a minimum 1200 ms loading animation in the dialog
+        // before it's content is rendered.
+        await waitFor(
+            () => expect(screen.queryByText('Unable to convert to a channel because group members are part of different teams')).toBeInTheDocument(),
+            {timeout: 1500},
+        );
+
+        expect(screen.queryByText('Select Team')).not.toBeInTheDocument();
+        expect(screen.queryByPlaceholderText('Channel name')).not.toBeInTheDocument();
+        expect(screen.queryByText('Edit')).not.toBeInTheDocument();
+    });
+
+    test('multiple common teams - trying conversion', async () => {
+        TestHelper.initBasic(Client4);
+        nock(Client4.getBaseRoute()).
+            get('/channels/channel_id_1/common_teams').
+            reply(200, [
+                {id: 'team_id_1', display_name: 'Team 1'},
+                {id: 'team_id_2', display_name: 'Team 2'},
+            ]);
+
+        baseProps.actions.convertGroupMessageToPrivateChannel.mockResolvedValueOnce({});
+
+        renderWithFullContext(
+            <ConvertGmToChannelModal {...baseProps}/>,
+            baseState,
+        );
+
+        // we need to use waitFor for first assertion as we have a minimum 1200 ms loading animation in the dialog
+        // before it's content is rendered.
+        await waitFor(
+            () => expect(screen.queryByText('Conversation history will be visible to any channel members')).toBeInTheDocument(),
+            {timeout: 1500},
+        );
+
+        const teamDropdown = screen.queryByText('Select Team');
+        expect(teamDropdown).not.toBeNull();
+        fireEvent(
+            teamDropdown!,
+            new MouseEvent('mousedown', {
+                bubbles: true,
+                cancelable: true,
+            }),
+        );
+
+        const team1Option = screen.queryByText('Team 1');
+        expect(team1Option).toBeInTheDocument();
+        fireEvent.click(team1Option!);
+
+        const channelNameInput = screen.queryByPlaceholderText('Channel name');
+        expect(channelNameInput).toBeInTheDocument();
+        fireEvent.change(channelNameInput!, {target: {value: 'Channel name set by me'}});
+
+        const confirmButton = screen.queryByText('Convert to private channel');
+        expect(channelNameInput).toBeInTheDocument();
+
+        await act(async () => {
+            fireEvent.click(confirmButton!);
+        });
+    });
+});

webapp/channels/src/components/convert_gm_to_channel_modal/convert_gm_to_channel_modal.tsx

@@ -0,0 +1,217 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import classNames from 'classnames';
+import type {ComponentProps} from 'react';
+import React, {useCallback, useEffect, useRef, useState} from 'react';
+import {useIntl} from 'react-intl';
+import {useDispatch} from 'react-redux';
+
+import './convert_gm_to_channel_modal.scss';
+
+import {GenericModal} from '@mattermost/components';
+import type {Channel} from '@mattermost/types/channels';
+import type {ServerError} from '@mattermost/types/errors';
+import type {Team} from '@mattermost/types/teams';
+import type {UserProfile} from '@mattermost/types/users';
+
+import type {ActionResult} from 'mattermost-redux/types/actions';
+import {displayUsername} from 'mattermost-redux/utils/user_utils';
+
+import {getGroupMessageMembersCommonTeams} from 'actions/team_actions';
+import {trackEvent} from 'actions/telemetry_actions';
+
+import ChannelNameFormField from 'components/channel_name_form_field/channel_name_form_field';
+import type {Actions} from 'components/convert_gm_to_channel_modal/index';
+import NoCommonTeamsError from 'components/convert_gm_to_channel_modal/no_common_teams/no_common_teams';
+import TeamSelector from 'components/convert_gm_to_channel_modal/team_selector/team_selector';
+import WarningTextSection from 'components/convert_gm_to_channel_modal/warning_text_section/warning_text_section';
+import LoadingSpinner from 'components/widgets/loading/loading_spinner';
+
+export type Props = {
+    onExited: () => void;
+    channel: Channel;
+    actions: Actions;
+    profilesInChannel: UserProfile[];
+    teammateNameDisplaySetting: string;
+    currentUserId: string;
+}
+
+const ConvertGmToChannelModal = (props: Props) => {
+    const intl = useIntl();
+    const {formatMessage} = intl;
+
+    const [channelName, setChannelName] = useState<string>('');
+    const channelURL = useRef<string>('');
+    const handleChannelURLChange = useCallback((newURL: string) => {
+        channelURL.current = newURL;
+    }, []);
+
+    const [channelMemberNames, setChannelMemberNames] = useState<string[]>([]);
+
+    useEffect(() => {
+        const validProfilesInChannel = props.profilesInChannel.
+            filter((user) => user.id !== props.currentUserId && user.delete_at === 0).
+            map((user) => displayUsername(user, props.teammateNameDisplaySetting));
+
+        setChannelMemberNames(validProfilesInChannel);
+    }, [props.profilesInChannel]);
+
+    const [commonTeamsById, setCommonTeamsById] = useState<{[id: string]: Team}>({});
+    const [commonTeamsFetched, setCommonTeamsFetched] = useState<boolean>(false);
+    const [loadingAnimationTimeout, setLoadingAnimationTimeout] = useState<boolean>(false);
+    const [selectedTeamId, setSelectedTeamId] = useState<string>();
+    const [nameError, setNameError] = useState<boolean>(false);
+    const [conversionError, setConversionError] = useState<string>();
+
+    const dispatch = useDispatch();
+
+    const mounted = useRef(false);
+    useEffect(() => {
+        mounted.current = true;
+        return (() => {
+            mounted.current = false;
+        });
+    }, []);
+
+    useEffect(() => {
+        const work = async () => {
+            const response = await dispatch(getGroupMessageMembersCommonTeams(props.channel.id)) as ActionResult<Team[], ServerError>;
+            if (!mounted.current) {
+                return;
+            }
+
+            if (response.error || !response.data) {
+                return;
+            }
+            const teams = response.data;
+
+            const teamsById: {[id: string]: Team} = {};
+            teams.forEach((team) => {
+                teamsById[team.id] = team;
+            });
+
+            setCommonTeamsById(teamsById);
+            setCommonTeamsFetched(true);
+
+            // if there is only common team, selected it.
+            if (teams.length === 1) {
+                setSelectedTeamId(teams[0].id);
+            }
+        };
+
+        work();
+        setTimeout(() => setLoadingAnimationTimeout(true), 1200);
+    }, []);
+
+    const handleConfirm = useCallback(async () => {
+        if (!selectedTeamId) {
+            return;
+        }
+
+        const {error} = await props.actions.convertGroupMessageToPrivateChannel(props.channel.id, selectedTeamId, channelName.trim(), channelURL.current.trim());
+
+        if (error) {
+            setConversionError(error.message);
+            return;
+        }
+
+        setConversionError(undefined);
+        trackEvent('actions', 'convert_group_message_to_private_channel', {channel_id: props.channel.id});
+        props.onExited();
+    }, [selectedTeamId, props.channel.id, channelName, channelURL.current, props.actions.moveChannelsInSidebar]);
+
+    const showLoader = !commonTeamsFetched || !loadingAnimationTimeout;
+    const canCreate = selectedTeamId !== undefined && channelName !== '' && !nameError;
+    const modalProps: Partial<ComponentProps<typeof GenericModal>> = {};
+    let modalBody;
+
+    if (!showLoader && Object.keys(commonTeamsById).length === 0) {
+        modalProps.confirmButtonText = formatMessage({id: 'generic.okay', defaultMessage: 'Okay'});
+        modalProps.handleConfirm = props.onExited;
+
+        modalBody = (
+            <div className='convert-gm-to-channel-modal-body error'>
+                <NoCommonTeamsError/>
+            </div>
+        );
+    } else {
+        modalProps.handleCancel = showLoader ? undefined : props.onExited;
+        modalProps.isDeleteModal = true;
+        modalProps.cancelButtonText = formatMessage({id: 'channel_modal.cancel', defaultMessage: 'Cancel'});
+        modalProps.confirmButtonText = formatMessage({id: 'sidebar_left.sidebar_channel_modal.confirmation_text', defaultMessage: 'Convert to private channel'});
+        modalProps.isConfirmDisabled = !canCreate;
+
+        let subBody;
+        if (showLoader) {
+            subBody = (
+                <div className='loadingIndicator'>
+                    <LoadingSpinner/>
+                </div>
+            );
+        } else {
+            subBody = (
+                <React.Fragment>
+                    <WarningTextSection channelMemberNames={channelMemberNames}/>
+
+                    {
+                        Object.keys(commonTeamsById).length > 1 &&
+                        <TeamSelector
+                            teamsById={commonTeamsById}
+                            onChange={setSelectedTeamId}
+                        />
+                    }
+
+                    <ChannelNameFormField
+                        value={channelName}
+                        name='convert-gm-to-channel-modal-channel-name'
+                        placeholder={formatMessage({id: 'sidebar_left.sidebar_channel_modal.channel_name_placeholder', defaultMessage: 'Enter a name for the channel'})}
+                        autoFocus={false}
+                        onDisplayNameChange={setChannelName}
+                        onURLChange={handleChannelURLChange}
+                        onErrorStateChange={setNameError}
+                    />
+
+                    {
+                        conversionError &&
+                        <div className='conversion-error'>
+                            <i className='icon icon-alert-outline'/>
+                            <span>{conversionError}</span>
+                        </div>
+                    }
+
+                </React.Fragment>
+            );
+        }
+
+        modalBody = (
+            <div
+                className={classNames({
+                    'convert-gm-to-channel-modal-body': true,
+                    loading: showLoader,
+                    'single-team': Object.keys(commonTeamsById).length === 1,
+                    'multi-team': Object.keys(commonTeamsById).length > 1,
+                })}
+            >
+                {subBody}
+            </div>
+        );
+    }
+
+    return (
+        <GenericModal
+            id='convert-gm-to-channel-modal'
+            className='convert-gm-to-channel-modal'
+            modalHeaderText={formatMessage({id: 'sidebar_left.sidebar_channel_modal.header', defaultMessage: 'Convert to Private Channel'})}
+            compassDesign={true}
+            handleConfirm={showLoader ? undefined : handleConfirm}
+            onExited={props.onExited}
+            autoCloseOnConfirmButton={false}
+            {...modalProps}
+        >
+            {modalBody}
+        </GenericModal>
+    );
+};
+
+export default ConvertGmToChannelModal;

webapp/channels/src/components/convert_gm_to_channel_modal/index.ts

@@ -0,0 +1,56 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import {connect} from 'react-redux';
+import type {ActionCreatorsMapObject, Dispatch} from 'redux';
+import {bindActionCreators} from 'redux';
+
+import {convertGroupMessageToPrivateChannel} from 'mattermost-redux/actions/channels';
+import {getTeammateNameDisplaySetting} from 'mattermost-redux/selectors/entities/preferences';
+import {
+    getCurrentUserId,
+    makeGetProfilesInChannel,
+} from 'mattermost-redux/selectors/entities/users';
+import type {Action, ActionResult} from 'mattermost-redux/types/actions';
+
+import {moveChannelsInSidebar} from 'actions/views/channel_sidebar';
+import {closeModal} from 'actions/views/modals';
+
+import type {Props} from 'components/convert_gm_to_channel_modal/convert_gm_to_channel_modal';
+import ConvertGmToChannelModal from 'components/convert_gm_to_channel_modal/convert_gm_to_channel_modal';
+
+import type {GlobalState} from 'types/store';
+
+function makeMapStateToProps() {
+    const getProfilesInChannel = makeGetProfilesInChannel();
+
+    return (state: GlobalState, ownProps: Props) => {
+        const allProfilesInChannel = getProfilesInChannel(state, ownProps.channel.id);
+        const currentUserId = getCurrentUserId(state);
+        const teammateNameDisplaySetting = getTeammateNameDisplaySetting(state);
+
+        return {
+            profilesInChannel: allProfilesInChannel,
+            teammateNameDisplaySetting,
+            currentUserId,
+        };
+    };
+}
+
+export type Actions = {
+    closeModal: (modalID: string) => void;
+    convertGroupMessageToPrivateChannel: (channelID: string, teamID: string, displayName: string, name: string) => Promise<ActionResult>;
+    moveChannelsInSidebar: (categoryId: string, targetIndex: number, draggableChannelId: string, setManualSorting?: boolean) => void;
+}
+
+function mapDispatchToProps(dispatch: Dispatch) {
+    return {
+        actions: bindActionCreators<ActionCreatorsMapObject<Action>, Actions>({
+            closeModal,
+            convertGroupMessageToPrivateChannel,
+            moveChannelsInSidebar,
+        }, dispatch),
+    };
+}
+
+export default connect(makeMapStateToProps, mapDispatchToProps)(ConvertGmToChannelModal);

webapp/channels/src/components/convert_gm_to_channel_modal/no_common_teams/no_common_teams.tsx

@@ -0,0 +1,31 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import React from 'react';
+import {FormattedMessage} from 'react-intl';
+
+import 'components/convert_gm_to_channel_modal/warning_text_section/warning_text_section.scss';
+
+const NoCommonTeamsError = (): JSX.Element => {
+    return (
+        <div className='warning-section error'>
+            <i className='fa fa-exclamation-circle'/>
+            <div className='warning-text'>
+                <div className='warning-header'>
+                    <FormattedMessage
+                        id='sidebar_left.sidebar_channel_modal.no_common_teams_error.heading'
+                        defaultMessage='Unable to convert to a channel because group members are part of different teams'
+                    />
+                </div>
+                <div className='warning-body'>
+                    <FormattedMessage
+                        id='sidebar_left.sidebar_channel_modal.no_common_teams_error.body'
+                        defaultMessage='Group Message cannot be converted to a channel because members are not a part of the same team. Add all members to a single team to convert this group message to a channel in that team.'
+                    />
+                </div>
+            </div>
+        </div>
+    );
+};
+
+export default NoCommonTeamsError;

webapp/channels/src/components/convert_gm_to_channel_modal/team_selector/team_selector.tsx

@@ -0,0 +1,51 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import React, {useCallback, useState} from 'react';
+import {useIntl} from 'react-intl';
+import {useSelector} from 'react-redux';
+
+import type {Team} from '@mattermost/types/teams';
+
+import {getCurrentLocale} from 'selectors/i18n';
+
+import DropdownInput from 'components/dropdown_input';
+
+export type Props = {
+    teamsById: {[id: string]: Team};
+    onChange: (teamId: string) => void;
+}
+
+const TeamSelector = (props: Props): JSX.Element => {
+    const [value, setValue] = useState<Team>();
+    const intl = useIntl();
+    const {formatMessage} = intl;
+
+    const handleTeamChange = useCallback((e) => {
+        const teamId = e.value as string;
+
+        setValue(props.teamsById[teamId]);
+        props.onChange(teamId);
+    }, []);
+
+    const currentLocale = useSelector(getCurrentLocale);
+
+    const teamValues = Object.values(props.teamsById).
+        map((team) => ({value: team.id, label: team.display_name})).
+        sort((teamA, teamB) => teamA.label.localeCompare(teamB.label, currentLocale));
+
+    return (
+        <DropdownInput
+            className='team_selector'
+            required={true}
+            onChange={handleTeamChange}
+            value={value ? {label: value.display_name, value: value.id} : undefined}
+            options={teamValues}
+            legend={formatMessage({id: 'sidebar_left.sidebar_channel_modal.select_team_placeholder', defaultMessage: 'Select Team'})}
+            placeholder={formatMessage({id: 'sidebar_left.sidebar_channel_modal.select_team_placeholder', defaultMessage: 'Select Team'})}
+            name='team_selector'
+        />
+    );
+};
+
+export default TeamSelector;

webapp/channels/src/components/convert_gm_to_channel_modal/warning_text_section/warning_text_section.scss

@@ -0,0 +1,35 @@
+.warning-section {
+    display: flex;
+    padding: 16px;
+    border: 1px solid rgba(var(--sidebar-text-active-border-rgb), 0.16);
+    background: rgba(var(--sidebar-text-active-border-rgb), 0.08);
+    border-radius: 4px;
+    gap: 12px;
+
+    &.error {
+        border: 1px solid rgba(var(--dnd-indicator-rgb), 0.16);
+        background: rgba(var(--dnd-indicator-rgb), 0.08);
+
+        .fa.fa-exclamation-circle {
+            color: rgba(var(--dnd-indicator-rgb), 1);
+        }
+    }
+
+    .fa.fa-exclamation-circle {
+        width: 24px;
+        height: 24px;
+        color: rgba(var(--sidebar-text-active-border-rgb), 1);
+        font-size: 24px;
+    }
+
+    .warning-text {
+        display: flex;
+        flex-direction: column;
+        color: var(--center-channel-color-88);
+        gap: 8px;
+    }
+
+    .warning-header {
+        font-weight: bold;
+    }
+}

webapp/channels/src/components/convert_gm_to_channel_modal/warning_text_section/warning_text_section.tsx

@@ -0,0 +1,46 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import React from 'react';
+import {FormattedMessage, useIntl} from 'react-intl';
+
+import 'components/convert_gm_to_channel_modal/warning_text_section/warning_text_section.scss';
+
+export type Props = {
+    channelMemberNames: string[];
+}
+const WarningTextSection = (props: Props): JSX.Element => {
+    const intl = useIntl();
+
+    let memberNames: string;
+
+    if (props.channelMemberNames.length > 0) {
+        memberNames = intl.formatList(props.channelMemberNames);
+    } else {
+        memberNames = intl.formatMessage({id: 'sidebar_left.sidebar_channel_modal.warning_body_yourself', defaultMessage: 'yourself'});
+    }
+
+    return (
+        <div className='warning-section'>
+            <i className='fa fa-exclamation-circle'/>
+            <div className='warning-text'>
+                <div className='warning-header'>
+                    <FormattedMessage
+                        id='sidebar_left.sidebar_channel_modal.warning_header'
+                        defaultMessage='Conversation history will be visible to any channel members'
+                    />
+                </div>
+                <div className='warning-body'>
+                    <FormattedMessage
+                        id='sidebar_left.sidebar_channel_modal.warning_body'
+                        defaultMessage='You are about to convert the Group Message with {memberNames} to a Channel. This cannot be undone.'
+                        values={{
+                            memberNames,
+                        }}
+                    />
+                </div>
+            </div>
+        </div>
+    );
+};
+export default WarningTextSection;

webapp/channels/src/components/dropdown_input.tsx

@@ -2,11 +2,17 @@
 // See LICENSE.txt for license information.
 
 import classNames from 'classnames';
-import React, {useState} from 'react';
+import React, {useRef, useState} from 'react';
 import type {CSSProperties} from 'react';
+import {useIntl} from 'react-intl';
 import ReactSelect, {components} from 'react-select';
 import type {Props as SelectProps, ActionMeta} from 'react-select';
 
+import InputError from 'components/input_error';
+import type {CustomMessageInputType} from 'components/widgets/inputs/input/input';
+
+import {ItemStatus} from 'utils/constants';
+
 import './dropdown_input.scss';
 
 // TODO: This component needs work, should not be used outside of AddressInfo until this comment is removed.
@@ -22,6 +28,7 @@ type Props<T> = Omit<SelectProps<T>, 'onChange'> & {
     error?: string;
     onChange: (value: T, action: ActionMeta<T>) => void;
     testId?: string;
+    required?: boolean;
 };
 
 const baseStyles = {
@@ -73,19 +80,6 @@ const Option = (props: any) => {
     );
 };
 
-const renderError = (error?: string) => {
-    if (!error) {
-        return null;
-    }
-
-    return (
-        <div className='Input___error'>
-            <i className='icon icon-alert-outline'/>
-            <span>{error}</span>
-        </div>
-    );
-};
-
 const DropdownInput = <T extends ValueType>(props: Props<T>) => {
     const {value, placeholder, className, addon, name, textPrefix, legend, onChange, styles, options, error, testId, ...otherProps} = props;
 
@@ -101,10 +95,29 @@ const DropdownInput = <T extends ValueType>(props: Props<T>) => {
         }
     };
 
+    const {formatMessage} = useIntl();
+    const [customInputLabel, setCustomInputLabel] = useState<CustomMessageInputType>(null);
+    const ownValue = useRef<T>();
+
+    const ownOnChange = (value: T, action: ActionMeta<T>) => {
+        ownValue.current = value;
+        onChange(value, action);
+    };
+    const validateInput = () => {
+        if (!props.required || (ownValue.current !== null && ownValue.current)) {
+            setCustomInputLabel(null);
+            return;
+        }
+
+        const validationErrorMsg = formatMessage({id: 'widget.input.required', defaultMessage: 'This field is required'});
+        setCustomInputLabel({type: ItemStatus.ERROR, value: validationErrorMsg});
+    };
+
     const onInputBlur = (event: React.FocusEvent<HTMLElement>) => {
         const {onBlur} = props;
 
         setFocused(false);
+        validateInput();
 
         if (onBlur) {
             onBlur(event);
@@ -112,6 +125,7 @@ const DropdownInput = <T extends ValueType>(props: Props<T>) => {
     };
 
     const showLegend = Boolean(focused || value);
+    const isError = error || customInputLabel?.type === 'error';
 
     return (
         <div
@@ -120,7 +134,7 @@ const DropdownInput = <T extends ValueType>(props: Props<T>) => {
         >
             <fieldset
                 className={classNames('Input_fieldset', className, {
-                    Input_fieldset___error: error,
+                    Input_fieldset___error: isError,
                     Input_fieldset___legend: showLegend,
                 })}
             >
@@ -145,14 +159,17 @@ const DropdownInput = <T extends ValueType>(props: Props<T>) => {
                         className={classNames('Input', className, {Input__focus: showLegend})}
                         classNamePrefix={'DropDown'}
                         value={value}
-                        onChange={onChange as any} // types are not working correctly for multiselect
+                        onChange={ownOnChange as any} // types are not working correctly for multiselect
                         styles={{...baseStyles, ...styles}}
                         {...otherProps}
                     />
                 </div>
                 {addon}
             </fieldset>
-            {renderError(error)}
+            <InputError
+                message={error}
+                custom={customInputLabel}
+            />
         </div>
     );
 };

webapp/channels/src/components/input_error.tsx

@@ -0,0 +1,42 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import classNames from 'classnames';
+import React from 'react';
+
+import type {CustomMessageInputType} from 'components/widgets/inputs/input/input';
+
+import {ItemStatus} from 'utils/constants';
+
+type Props = {
+    message?: string;
+    custom?: CustomMessageInputType;
+}
+
+const InputError = (props: Props) => {
+    if (props.message) {
+        return (
+            <div className='Input___error'>
+                <i className='icon icon-alert-outline'/>
+                <span>{props.message}</span>
+            </div>
+        );
+    } else if (props.custom) {
+        return (
+            <div className={`Input___customMessage Input___${props.custom.type}`}>
+                <i
+                    className={classNames(`icon ${props.custom.type}`, {
+                        'icon-alert-outline': props.custom.type === ItemStatus.WARNING,
+                        'icon-alert-circle-outline': props.custom.type === ItemStatus.ERROR,
+                        'icon-information-outline': props.custom.type === ItemStatus.INFO,
+                        'icon-check': props.custom.type === ItemStatus.SUCCESS,
+                    })}
+                />
+                <span>{props.custom.value}</span>
+            </div>
+        );
+    }
+    return null;
+};
+
+export default InputError;

webapp/channels/src/components/new_channel_modal/new_channel_modal.scss

@@ -1,10 +1,4 @@
 .new-channel-modal {
-    .new-channel-modal-name-input {
-        height: 34px !important;
-        border: 0 !important;
-        border-radius: 0 !important;
-    }
-
     .new-channel-modal-type-selector {
         margin-top: 24px;
     }

webapp/channels/src/components/new_channel_modal/new_channel_modal.tsx

@@ -2,8 +2,7 @@
 // See LICENSE.txt for license information.
 
 import classNames from 'classnames';
-import crypto from 'crypto';
-import React, {useState} from 'react';
+import React, {useCallback, useState} from 'react';
 import {Tooltip} from 'react-bootstrap';
 import {FormattedMessage, useIntl} from 'react-intl';
 import {useDispatch, useSelector} from 'react-redux';
@@ -25,15 +24,12 @@ import type {DispatchFunc} from 'mattermost-redux/types/actions';
 import {switchToChannel} from 'actions/views/channel';
 import {closeModal} from 'actions/views/modals';
 
+import ChannelNameFormField from 'components/channel_name_form_field/channel_name_form_field';
 import OverlayTrigger from 'components/overlay_trigger';
-import Input from 'components/widgets/inputs/input/input';
-import URLInput from 'components/widgets/inputs/url_input/url_input';
 import PublicPrivateSelector from 'components/widgets/public-private-selector/public-private-selector';
 
 import Pluggable from 'plugins/pluggable';
-import Constants, {ItemStatus, ModalIdentifiers} from 'utils/constants';
-import {cleanUpUrlable, validateChannelUrl, getSiteURL} from 'utils/url';
-import {localizeMessage} from 'utils/utils';
+import Constants, {ModalIdentifiers} from 'utils/constants';
 
 import type {GlobalState} from 'types/store';
 
@@ -53,20 +49,6 @@ export function getChannelTypeFromPermissions(canCreatePublicChannel: boolean, c
     return channelType as ChannelType;
 }
 
-export function validateDisplayName(displayName: string) {
-    const errors: string[] = [];
-
-    if (displayName.length < Constants.MIN_CHANNELNAME_LENGTH) {
-        errors.push(localizeMessage('channel_modal.name.longer', 'Channel names must have at least 2 characters.'));
-    }
-
-    if (displayName.length > Constants.MAX_CHANNELNAME_LENGTH) {
-        errors.push(localizeMessage('channel_modal.name.shorter', 'Channel names must have maximum 64 characters.'));
-    }
-
-    return errors;
-}
-
 const enum ServerErrorId {
     CHANNEL_URL_SIZE = 'model.channel.is_valid.1_or_more.app_error',
     CHANNEL_UPDATE_EXISTS = 'store.sql_channel.update.exists.app_error',
@@ -78,7 +60,7 @@ const NewChannelModal = () => {
     const intl = useIntl();
     const {formatMessage} = intl;
 
-    const {id: currentTeamId, name: currentTeamName} = useSelector((state: GlobalState) => getCurrentTeam(state));
+    const {id: currentTeamId} = useSelector(getCurrentTeam);
 
     const canCreatePublicChannel = useSelector((state: GlobalState) => (currentTeamId ? haveICurrentChannelPermission(state, Permissions.CREATE_PUBLIC_CHANNEL) : false));
     const canCreatePrivateChannel = useSelector((state: GlobalState) => (currentTeamId ? haveICurrentChannelPermission(state, Permissions.CREATE_PRIVATE_CHANNEL) : false));
@@ -88,12 +70,10 @@ const NewChannelModal = () => {
     const [displayName, setDisplayName] = useState('');
     const [url, setURL] = useState('');
     const [purpose, setPurpose] = useState('');
-    const [displayNameModified, setDisplayNameModified] = useState(false);
-    const [urlModified, setURLModified] = useState(false);
-    const [displayNameError, setDisplayNameError] = useState('');
     const [urlError, setURLError] = useState('');
     const [purposeError, setPurposeError] = useState('');
     const [serverError, setServerError] = useState('');
+    const [channelInputError, setChannelInputError] = useState(false);
 
     // create a board along with the channel
     const pluginsComponentsList = useSelector((state: GlobalState) => state.plugins.components);
@@ -215,48 +195,10 @@ const NewChannelModal = () => {
         }
     };
 
-    const handleOnDisplayNameChange = (e: React.ChangeEvent<HTMLInputElement>) => {
-        e.preventDefault();
-        const {target: {value: displayName}} = e;
-
-        const displayNameErrors = validateDisplayName(displayName);
-
-        setDisplayNameError(displayNameErrors.length ? displayNameErrors[displayNameErrors.length - 1] : '');
-        setDisplayName(displayName);
-        setServerError('');
-
-        if (!urlModified) {
-            setURL(cleanUpUrlable(displayName));
-            setURLError('');
-        }
-    };
-
-    const handleOnDisplayNameBlur = () => {
-        if (displayName && !url) {
-            setURL(crypto.randomBytes(16).toString('hex'));
-        }
-        if (!displayNameModified) {
-            setDisplayNameModified(true);
-        }
-    };
-
-    const handleOnURLChange = (e: React.ChangeEvent<HTMLInputElement>) => {
-        e.preventDefault();
-        const {target: {value: url}} = e;
-
-        const cleanURL = url.toLowerCase().replace(/\s/g, '-');
-        const urlErrors = validateChannelUrl(cleanURL, intl) as string[];
-
-        setURLError(urlErrors.length ? urlErrors[urlErrors.length - 1] : '');
-        setURL(cleanURL);
-        setURLModified(true);
-        setServerError('');
-    };
-
-    const handleOnTypeChange = (channelType: ChannelType) => {
+    const handleOnTypeChange = useCallback((channelType: ChannelType) => {
         setType(channelType);
         setServerError('');
-    };
+    }, []);
 
     const handleOnPurposeChange = (e: React.ChangeEvent<HTMLTextAreaElement>) => {
         e.preventDefault();
@@ -272,7 +214,7 @@ const NewChannelModal = () => {
         e.stopPropagation();
     };
 
-    const canCreate = displayName && !displayNameError && !urlError && type && !purposeError && !serverError && canCreateFromPluggable;
+    const canCreate = displayName && !urlError && type && !purposeError && !serverError && canCreateFromPluggable && !channelInputError;
 
     const newBoardInfoIcon = (
         <OverlayTrigger
@@ -318,31 +260,13 @@ const NewChannelModal = () => {
             onExited={handleOnModalCancel}
         >
             <div className='new-channel-modal-body'>
-                <Input
-                    type='text'
-                    autoComplete='off'
-                    autoFocus={true}
-                    required={true}
-                    name='new-channel-modal-name'
-                    containerClassName='new-channel-modal-name-container'
-                    inputClassName='new-channel-modal-name-input'
-                    label={formatMessage({id: 'channel_modal.name.label', defaultMessage: 'Channel name'})}
-                    placeholder={formatMessage({id: 'channel_modal.name.placeholder', defaultMessage: 'Enter a name for your new channel'})}
-                    limit={Constants.MAX_CHANNELNAME_LENGTH}
+                <ChannelNameFormField
                     value={displayName}
-                    customMessage={displayNameModified ? {type: ItemStatus.ERROR, value: displayNameError} : null}
-                    onChange={handleOnDisplayNameChange}
-                    onBlur={handleOnDisplayNameBlur}
-                />
-                <URLInput
-                    className='new-channel-modal__url'
-                    base={getSiteURL()}
-                    path={`${currentTeamName}/channels`}
-                    pathInfo={url}
-                    limit={Constants.MAX_CHANNELNAME_LENGTH}
-                    shortenLength={Constants.DEFAULT_CHANNELURL_SHORTEN_LENGTH}
-                    error={urlError}
-                    onChange={handleOnURLChange}
+                    name='new-channel-modal-name'
+                    placeholder={formatMessage({id: 'channel_modal.name.placeholder', defaultMessage: 'Enter a name for your new channel'})}
+                    onDisplayNameChange={setDisplayName}
+                    onURLChange={setURL}
+                    onErrorStateChange={setChannelInputError}
                 />
                 <PublicPrivateSelector
                     className='new-channel-modal-type-selector'

webapp/channels/src/components/post_markdown/system_message_helpers.tsx

@@ -14,12 +14,13 @@ import {isPostEphemeral} from 'mattermost-redux/utils/post_utils';
 
 import Markdown from 'components/markdown';
 import CombinedSystemMessage from 'components/post_view/combined_system_message';
+import GMConversionMessage from 'components/post_view/gm_conversion_message/gm_conversion_message';
 import PostAddChannelMember from 'components/post_view/post_add_channel_member';
 
 import type {TextFormattingOptions} from 'utils/text_formatting';
 import {getSiteURL} from 'utils/url';
 
-function renderUsername(value: string): ReactNode {
+export function renderUsername(value: string): ReactNode {
     const username = (value[0] === '@') ? value : `@${value}`;
 
     const options = {
@@ -426,6 +427,13 @@ export function renderSystemMessage(post: Post, currentTeam: Team, channel: Chan
                 messageData={messageData}
             />
         );
+    } else if (post.type === Posts.POST_TYPES.GM_CONVERTED_TO_CHANNEL) {
+        // This is rendered via a separate component instead of registering in
+        // systemMessageRenderers because we need to format a list with keeping i18n support
+        // which cannot be done outside a react component.
+        return (
+            <GMConversionMessage post={post}/>
+        );
     }
 
     return null;

webapp/channels/src/components/post_view/gm_conversion_message/gm_conversion_message.tsx

@@ -0,0 +1,60 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import React, {useEffect, useRef} from 'react';
+import {FormattedMessage, useIntl} from 'react-intl';
+import {useDispatch, useSelector} from 'react-redux';
+
+import type {Post} from '@mattermost/types/posts';
+
+import {getMissingProfilesByIds} from 'mattermost-redux/actions/users';
+import {makeGetProfilesByIdsAndUsernames} from 'mattermost-redux/selectors/entities/users';
+
+import {renderUsername} from 'components/post_markdown/system_message_helpers';
+
+import type {GlobalState} from 'types/store';
+
+export type Props = {
+    post: Post;
+}
+function GMConversionMessage(props: Props): JSX.Element {
+    const convertedByUserId = props.post.props.convertedByUserId;
+    const gmMembersDuringConversionIDs = props.post.props.gmMembersDuringConversionIDs as string[];
+
+    const dispatch = useDispatch();
+    const intl = useIntl();
+
+    useEffect(() => {
+        dispatch(getMissingProfilesByIds(gmMembersDuringConversionIDs));
+    }, [props.post]);
+
+    const getProfilesByIdsAndUsernames = useRef(makeGetProfilesByIdsAndUsernames());
+    const userProfiles = useSelector(
+        (state: GlobalState) => getProfilesByIdsAndUsernames.current(
+            state,
+            {allUserIds: gmMembersDuringConversionIDs, allUsernames: []},
+        ),
+    );
+
+    const convertedByUserUsername = userProfiles.find((user) => user.id === convertedByUserId)!.username;
+    const gmMembersUsernames = userProfiles.map((user) => renderUsername(user.username));
+
+    if (!convertedByUserId || !gmMembersDuringConversionIDs || gmMembersDuringConversionIDs.length === 0) {
+        return (
+            <span>{props.post.message}</span>
+        );
+    }
+
+    return (
+        <FormattedMessage
+            id='api.channel.group_message_converted_to.private_channel'
+            defaultMessage='{convertedBy} created this channel from a group message with {gmMembers}.'
+            values={{
+                convertedBy: renderUsername(convertedByUserUsername),
+                gmMembers: intl.formatList(gmMembersUsernames),
+            }}
+        />
+    );
+}
+
+export default GMConversionMessage;

webapp/channels/src/i18n/en.json

@@ -2661,6 +2661,7 @@
   "api.channel.add_guest.added": "{addedUsername} added to the channel as a guest by {username}.",
   "api.channel.add_member.added": "{addedUsername} added to the channel by {username}.",
   "api.channel.delete_channel.archived": "{username} archived the channel.",
+  "api.channel.group_message_converted_to.private_channel": "{convertedBy} created this channel from a group message with {gmMembers}.",
   "api.channel.guest_join_channel.post_and_forget": "{username} joined the channel as a guest.",
   "api.channel.join_channel.post_and_forget": "{username} joined the channel.",
   "api.channel.leave.left": "{username} left the channel.",
@@ -3614,6 +3615,7 @@
   "generic.close": "Close",
   "generic.done": "Done",
   "generic.next": "Next",
+  "generic.okay": "Okay",
   "generic.previous": "Previous",
   "get_app.continueToBrowser": "View in Browser",
   "get_app.dontHaveTheDesktopApp": "Don't have the Desktop App?",
@@ -4822,6 +4824,7 @@
   "sidebar_left.sidebar_category_menu.viewCategory": "Mark category as read",
   "sidebar_left.sidebar_category.newDropBoxLabel": "Drag channels here...",
   "sidebar_left.sidebar_category.newLabel": "new",
+  "sidebar_left.sidebar_channel_menu_convert_to_channel": "Convert to Private Channel",
   "sidebar_left.sidebar_channel_menu.addMembers": "Add Members",
   "sidebar_left.sidebar_channel_menu.channels": "Channels",
   "sidebar_left.sidebar_channel_menu.copyLink": "Copy Link",
@@ -4841,6 +4844,15 @@
   "sidebar_left.sidebar_channel_menu.unfavoriteChannel": "Unfavorite",
   "sidebar_left.sidebar_channel_menu.unmuteChannel": "Unmute Channel",
   "sidebar_left.sidebar_channel_menu.unmuteConversation": "Unmute Conversation",
+  "sidebar_left.sidebar_channel_modal.channel_name_placeholder": "Enter a name for the channel",
+  "sidebar_left.sidebar_channel_modal.confirmation_text": "Convert to private channel",
+  "sidebar_left.sidebar_channel_modal.header": "Convert to Private Channel",
+  "sidebar_left.sidebar_channel_modal.no_common_teams_error.body": "Group Message cannot be converted to a channel because members are not a part of the same team. Add all members to a single team to convert this group message to a channel in that team.",
+  "sidebar_left.sidebar_channel_modal.no_common_teams_error.heading": "Unable to convert to a channel because group members are part of different teams",
+  "sidebar_left.sidebar_channel_modal.select_team_placeholder": "Select Team",
+  "sidebar_left.sidebar_channel_modal.warning_body": "You are about to convert the Group Message with {memberNames} to a Channel. This cannot be undone.",
+  "sidebar_left.sidebar_channel_modal.warning_body_yourself": "yourself",
+  "sidebar_left.sidebar_channel_modal.warning_header": "Conversation history will be visible to any channel members",
   "sidebar_left.sidebar_channel_navigator.addChannelsCta": "Add channels",
   "sidebar_left.sidebar_channel_navigator.inviteUsers": "Invite Users",
   "sidebar_left.sidebar_channel.selectedCount": "{count} selected",

webapp/channels/src/packages/mattermost-redux/src/actions/channels.ts

@@ -339,6 +339,40 @@ export function updateChannelPrivacy(channelId: string, privacy: string): Action
     };
 }
 
+export function convertGroupMessageToPrivateChannel(channelID: string, teamID: string, displayName: string, name: string): ActionFunc {
+    return async (dispatch: DispatchFunc, getState: GetStateFunc) => {
+        dispatch({type: ChannelTypes.UPDATE_CHANNEL_REQUEST, data: null});
+
+        let updatedChannel;
+        try {
+            updatedChannel = await Client4.convertGroupMessageToPrivateChannel(channelID, teamID, displayName, name);
+        } catch (error) {
+            forceLogoutIfNecessary(error, dispatch, getState);
+            dispatch({type: ChannelTypes.UPDATE_CHANNEL_FAILURE, error});
+            dispatch(logError(error));
+            return {error};
+        }
+
+        dispatch(batchActions([
+            {
+                type: ChannelTypes.RECEIVED_CHANNEL,
+                data: updatedChannel,
+            },
+            {
+                type: ChannelTypes.UPDATE_CHANNEL_SUCCESS,
+            },
+        ]));
+
+        // move the channel from direct message category to the default "channels" category
+        const channelsCategory = getCategoryInTeamByType(getState(), teamID, CategoryTypes.CHANNELS);
+        if (!channelsCategory) {
+            return {};
+        }
+
+        return updatedChannel;
+    };
+}
+
 export function updateChannelNotifyProps(userId: string, channelId: string, props: Partial<ChannelNotifyProps>): ActionFunc {
     return async (dispatch: DispatchFunc, getState: GetStateFunc) => {
         const notifyProps = {

webapp/channels/src/packages/mattermost-redux/src/constants/posts.ts

@@ -32,6 +32,7 @@ export const PostTypes = {
     ADD_BOT_TEAMS_CHANNELS: 'add_bot_teams_channels' as PostType,
     SYSTEM_WARN_METRIC_STATUS: 'warn_metric_status' as PostType,
     REMINDER: 'reminder' as PostType,
+    GM_CONVERTED_TO_CHANNEL: 'system_gm_to_channel' as PostType,
 };
 
 export default {

webapp/channels/src/packages/mattermost-redux/src/selectors/entities/teams.ts

@@ -143,6 +143,11 @@ export const getCurrentRelativeTeamUrl: (state: GlobalState) => string = createS
     },
 );
 
+export function getRelativeTeamUrl(state: GlobalState, teamId: string): string {
+    const team = getTeam(state, teamId);
+    return `/${team.name}`;
+}
+
 export const getCurrentTeamStats: (state: GlobalState) => TeamStats = createSelector(
     'getCurrentTeamStats',
     getCurrentTeamId,

webapp/channels/src/utils/constants.tsx

@@ -445,6 +445,7 @@ export const ModalIdentifiers = {
     SELF_HOSTED_EXPANSION: 'self_hosted_expansion',
     START_TRIAL_FORM_MODAL: 'start_trial_form_modal',
     START_TRIAL_FORM_MODAL_RESULT: 'start_trial_form_modal_result',
+    CONVERT_GM_TO_CHANNEL: 'convert_gm_to_channel',
 };
 
 export const UserStatuses = {

webapp/channels/src/utils/utils.tsx

@@ -2,6 +2,7 @@
 // See LICENSE.txt for license information.
 
 import {getName} from 'country-list';
+import crypto from 'crypto';
 import cssVars from 'css-vars-ponyfill';
 import type {Locale} from 'date-fns';
 import {isNil} from 'lodash';
@@ -1747,6 +1748,9 @@ export function getBlankAddressWithCountry(country?: string): Address {
     };
 }
 
+export function generateSlug(): string {
+    return crypto.randomBytes(16).toString('hex');
+}
 export function sortUsersAndGroups(a: UserProfile | Group, b: UserProfile | Group) {
     let aSortString = '';
     let bSortString = '';

webapp/platform/client/src/client4.ts

@@ -4201,6 +4201,26 @@ export default class Client4 {
             {method: 'delete', body: JSON.stringify(deletionRequest)},
         );
     }
+
+    getGroupMessageMembersCommonTeams = (channelId: string) => {
+        return this.doFetchWithResponse<Team[]>(
+            `${this.getChannelRoute(channelId)}/common_teams`,
+            {method: 'get'},
+        )
+    }
+
+    convertGroupMessageToPrivateChannel = (channelId: string, teamId: string, displayName: string, name: string) => {
+        const body = {
+            channel_id: channelId,
+            team_id: teamId,
+            display_name: displayName,
+            name: name,
+        }
+        return this.doFetchWithResponse<Channel>(
+            `${this.getChannelRoute(channelId)}/convert_to_channel?team_id=${teamId}`,
+            {method: 'post', body: JSON.stringify(body)},
+        )
+    }
 }
 
 export function parseAndMergeNestedHeaders(originalHeaders: any) {