UX: improve logic for new topic and new PM composer actions (#35834)

A follow-up to #35764.

This change enhances the logic when **New topic** option should be
shown/hidden in composer actions:

1. After user switches to **Personal message** mode, we allow to switch
back to **New topic** mode:
<img width="265" height="153" alt="Screenshot 2025-11-05 at 22 17 16"
src="https://github.com/user-attachments/assets/5dedf69d-8e0c-4b28-abb5-1be4d5662a81"
/>

and

<img width="332" height="177" alt="Screenshot 2025-11-05 at 22 22 05"
src="https://github.com/user-attachments/assets/5e17c82d-10fb-4407-b9f8-ced78a85fdd5"
/>

2. In **New topic** mode there is no need to render **New topic**
option:

|Before|After|
|---|---|
|<img width="345" height="233" alt="Screenshot 2025-11-05 at 18 57 20"
src="https://github.com/user-attachments/assets/eef3d4da-b58e-4a11-90df-f76774630fad"
/>|<img width="453" height="197" alt="Screenshot 2025-11-05 at 22 19 37"
src="https://github.com/user-attachments/assets/816b1199-01a7-4022-8695-61cc9edc2473"
/>|

config/locales/client.en.yml

@@ -2998,7 +2998,7 @@ en:
       reply: "Reply"
       cancel: "Cancel"
       create_topic: "Create Topic"
-      create_pm: "Send Message"
+      create_pm: "Send message"
       create_whisper: "Whisper"
       create_shared_draft: "Create Shared Draft"
       edit_shared_draft: "Edit Shared Draft"

frontend/discourse/tests/acceptance/composer-test.js

@@ -10,7 +10,7 @@ import {
   visit,
   waitFor,
 } from "@ember/test-helpers";
-import { test } from "qunit";
+import { module, test } from "qunit";
 import sinon from "sinon";
 import { PLATFORM_KEY_MODIFIER } from "discourse/lib/keyboard-shortcuts";
 import LinkLookup from "discourse/lib/link-lookup";
@@ -814,6 +814,51 @@ import { i18n } from "discourse-i18n";
           .exists("goes back to open state if there's errors");
       });
 
+      module(
+        "Composer can switch between new topic and new PM in different contexts",
+        function () {
+          test("within post/topic context", async function (assert) {
+            await visit("/t/this-is-a-test-topic/54081");
+            await click(".topic-post[data-post-number='1'] button.reply");
+            await selectKit(".composer-actions").expand();
+            assert.notStrictEqual(
+              selectKit(".composer-actions")
+                .rowByValue("create_private_message")
+                .exists(),
+              "New message option is not present when in reply mode"
+            );
+
+            await click("#reply-control .discard-button");
+            await visit("/");
+            await click("#create-topic");
+            await selectKit(".composer-actions").expand();
+            assert.true(
+              selectKit(".composer-actions")
+                .rowByValue("reply_to_topic")
+                .exists(),
+              "composer topic context is preserved when reopened"
+            );
+
+            await selectKit(".composer-actions").selectRowByValue(
+              "create_private_message"
+            );
+            assert.dom(".action-title").hasText(i18n("topic.private_message"));
+            assert
+              .dom(".save-or-cancel button")
+              .hasText(i18n("composer.create_pm"));
+
+            await selectKit(".composer-actions").expand();
+            await selectKit(".composer-actions").selectRowByValue(
+              "create_topic"
+            );
+            assert.dom(".action-title").hasText(i18n("topic.create_long"));
+            assert
+              .dom(".save-or-cancel button")
+              .hasText(i18n("composer.create_topic"));
+          });
+        }
+      );
+
       test("Composer can toggle between reply and createTopic", async function (assert) {
         await visit("/t/this-is-a-test-topic/54081");
         await click(".topic-post[data-post-number='1'] button.reply");

frontend/select-kit/addon/components/composer-actions.js

@@ -240,7 +240,11 @@ export default class ComposerActions extends DropdownSelectBoxComponent {
       });
     }
 
-    if (items.length === 0 && this.currentUser.can_create_topic) {
+    if (
+      this.currentUser.can_create_topic &&
+      this.action !== CREATE_TOPIC &&
+      (items.length === 0 || this.action === PRIVATE_MESSAGE)
+    ) {
       items.push({
         name: i18n("composer.composer_actions.create_topic.label"),
         description: i18n("composer.composer_actions.create_topic.desc"),

spec/system/private_message_spec.rb

@@ -30,7 +30,7 @@ describe "Private Message", type: :system do
       expect(composer).to be_opened
 
       composer.open_composer_actions
-      composer.select_action("New message")
+      composer.select_action(I18n.t("js.composer.composer_actions.create_personal_message.label"))
 
       expect(composer.button_label).to have_text(I18n.t("js.composer.create_pm"))
     end
@@ -42,7 +42,24 @@ describe "Private Message", type: :system do
       expect(composer).to be_opened
 
       composer.open_composer_actions
-      expect(composer).to have_no_action("New message")
+      expect(composer).to have_no_action(
+        I18n.t("js.composer.composer_actions.create_personal_message.label"),
+      )
+    end
+
+    it "can switch between topic and personal message modes" do
+      visit "/new-topic"
+      expect(composer).to be_opened
+
+      # Switch to personal message
+      composer.open_composer_actions
+      composer.select_action(I18n.t("js.composer.composer_actions.create_personal_message.label"))
+      expect(composer.button_label).to have_text(I18n.t("js.composer.create_pm"))
+
+      # Switch back to topic
+      composer.open_composer_actions
+      composer.select_action(I18n.t("js.composer.composer_actions.create_topic.label"))
+      expect(composer.button_label).to have_text(I18n.t("js.composer.create_topic"))
     end
   end
 end