fix(mouse): mouse=n should not adjust visual selection

Problem:

When mouse=n is set
  - Dragging the mouse enters visual mode, and then stops listening for
    mouse events.
  - Double/Triple/Quad clicking performs selections.
  - Clicking in visual mode moves the cursor (though not through the TUI).

Solution:

Explicitly gate mouse actions that affect visual mode with a check for
MOUSE_VISUAL. This matches the behavior described in :help mouse.

> If enabled for "v" (Visual mode) then double-click selects word-wise,
> triple-click makes it line-wise, and quadruple-click makes it
> rectangular block-wise.

(cherry picked from commit 24f7182390)

src/nvim/mouse.c

@@ -609,9 +609,10 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
 
   pos_T end_visual = { 0 };
   pos_T start_visual = { 0 };
+  bool mouse_can_visual = ui_mouse_has(MOUSE_VISUAL);
   if ((State & (MODE_NORMAL | MODE_INSERT))
       && !(mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))) {
-    if (which_button == MOUSE_LEFT) {
+    if (which_button == MOUSE_LEFT && mouse_can_visual) {
       if (is_click) {
         // stop Visual mode for a left click in a window, but not when on a status line
         if (VIsual_active) {
@@ -620,7 +621,7 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
       } else {
         jump_flags |= MOUSE_MAY_VIS;
       }
-    } else if (which_button == MOUSE_RIGHT) {
+    } else if (which_button == MOUSE_RIGHT && mouse_can_visual) {
       if (is_click && VIsual_active) {
         // Remember the start and end of visual before moving the cursor.
         if (lt(curwin->w_cursor, VIsual)) {
@@ -631,8 +632,10 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
           end_visual = curwin->w_cursor;
         }
       }
-      jump_flags |= MOUSE_FOCUS;
       jump_flags |= MOUSE_MAY_VIS;
+      jump_flags |= MOUSE_FOCUS;
+    } else if (which_button == MOUSE_RIGHT) {
+      jump_flags |= MOUSE_FOCUS;
     }
   }
 
@@ -650,7 +653,12 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
   // JUMP!
   int old_active = VIsual_active;
   pos_T save_cursor = curwin->w_cursor;
-  jump_flags = jump_to_mouse(jump_flags, oap == NULL ? NULL : &(oap->inclusive), which_button);
+
+  // Even though we gate *_VIS flags above, we want to make sure the cursor doesn't move
+  // in visual mode unless it is set as a mouse option
+  if (!VIsual_active || mouse_can_visual) {
+    jump_flags = jump_to_mouse(jump_flags, oap == NULL ? NULL : &(oap->inclusive), which_button);
+  }
 
   bool moved = (jump_flags & CURSOR_MOVED);
   bool in_winbar = (jump_flags & MOUSE_WINBAR);
@@ -891,7 +899,8 @@ bool do_mouse(oparg_T *oap, int c, int dir, int count, bool fixindent)
   } else if (in_status_line || in_sep_line) {
     // Do nothing if on status line or vertical separator
     // Handle double clicks otherwise
-  } else if ((mod_mask & MOD_MASK_MULTI_CLICK) && (State & (MODE_NORMAL | MODE_INSERT))) {
+  } else if ((mod_mask & MOD_MASK_MULTI_CLICK) && (State & (MODE_NORMAL | MODE_INSERT))
+             && mouse_can_visual) {
     if (is_click || !VIsual_active) {
       if (VIsual_active) {
         orig_cursor = VIsual;

src/nvim/ui.c

@@ -630,32 +630,44 @@ void ui_check_mouse(void)
     checkfor = ' ';  // don't use mouse for ":!cmd"
   }
 
-  // mouse should be active if at least one of the following is true:
+  if (ui_mouse_has(checkfor)) {
-  // - "c" is in 'mouse', or
+    has_mouse = true;
-  // - 'a' is in 'mouse' and "c" is in MOUSE_A, or
+  }
-  // - the current buffer is a help file and 'h' is in 'mouse' and we are in a
+}
-  //   normal editing mode (not at hit-return message).
+
+// check if 'mouse' is active for the given mode
+//
+// mouse should be active if at least one of the following is true:
+// - "mode" is in 'mouse', or
+// - 'a' is in 'mouse' and "mode" is in MOUSE_A, or
+// - the current buffer is a help file and 'h' is in 'mouse' and we are in a
+//   normal editing mode (not at hit-return message).
+bool ui_mouse_has(int mode)
+{
   for (char *p = p_mouse; *p; p++) {
     switch (*p) {
     case 'a':
-      if (vim_strchr(MOUSE_A, checkfor) != NULL) {
+      if (vim_strchr(MOUSE_A, mode) != NULL) {
-        has_mouse = true;
+        return true;
-        return;
       }
+
       break;
     case MOUSE_HELP:
-      if (checkfor != MOUSE_RETURN && curbuf->b_help) {
+      if (mode != MOUSE_RETURN && curbuf->b_help) {
-        has_mouse = true;
+        return true;
-        return;
       }
+
       break;
     default:
-      if (checkfor == *p) {
+      if (mode == *p) {
-        has_mouse = true;
+        return true;
-        return;
       }
+
+      break;
     }
   }
+
+  return false;
 }
 
 /// Check if current mode has changed.

test/functional/ui/mouse_spec.lua

@@ -111,6 +111,63 @@ describe('ui/mouse/input', function()
       })
     end)
 
+    it('double left click stays in normal mode if mouse does not contain v', function()
+      api.nvim_set_option_value('mouse', 'n', {})
+
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      screen:expect({
+        any = {
+          '%^testing',
+          'mouse',
+          'support and selection',
+        },
+        mode = 'normal',
+      })
+    end)
+
+    it('triple left click stays in normal mode if mouse does not contain v', function()
+      api.nvim_set_option_value('mouse', 'n', {})
+
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      screen:expect({
+        any = {
+          '%^testing',
+          'mouse',
+          'support and selection',
+        },
+        mode = 'normal',
+      })
+    end)
+
+    it('quadruple left click stays in normal mode if mouse does not contain v', function()
+      api.nvim_set_option_value('mouse', 'n', {})
+
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      feed('<LeftMouse><0,0>')
+      feed('<LeftRelease><0,0>')
+      screen:expect({
+        any = {
+          '%^testing',
+          'mouse',
+          'support and selection',
+        },
+        mode = 'normal',
+      })
+    end)
+
     describe('tab drag', function()
       it('in tabline on filler space moves tab to the end', function()
         feed_command('%delete')
@@ -579,6 +636,104 @@ describe('ui/mouse/input', function()
       })
     end)
 
+    it('left drag moves cursor if mouse does not contain v', function()
+      api.nvim_set_option_value('mouse', 'n', {})
+
+      -- drag events must be preceded by a click
+      feed('<LeftMouse><2,1>')
+      screen:expect({
+        any = {
+          'testing',
+          'mo%^use',
+          'support and selection',
+        },
+        mode = 'normal',
+      })
+      feed('<LeftDrag><4,1>')
+      screen:expect({
+        any = {
+          'testing',
+          'mous%^e',
+          'support and selection',
+        },
+        mode = 'normal',
+      })
+      feed('<LeftDrag><2,2>')
+      screen:expect({
+        any = {
+          'testing',
+          'mouse',
+          'su%^pport and selection',
+        },
+        mode = 'normal',
+      })
+      feed('<LeftDrag><0,0>')
+      screen:expect({
+        any = {
+          '%^testing',
+          'mouse',
+          'support and selection',
+        },
+        mode = 'normal',
+      })
+    end)
+
+    it('left drag does not adjust existing visual selection if mouse does not contain v', function()
+      api.nvim_set_option_value('mouse', 'n', {})
+
+      feed('gg^vlj')
+      screen:expect({
+        any = {
+          '{17:testing}',
+          '{17:m}%^ouse',
+          'support and selection',
+          'VISUAL',
+        },
+      })
+
+      -- drag events must be preceded by a click
+      feed('<LeftMouse><2,1>')
+      screen:expect({
+        any = {
+          '{17:testing}',
+          '{17:m}%^ouse',
+          'support and selection',
+          'VISUAL',
+        },
+        unchanged = true,
+      })
+      feed('<LeftDrag><4,1>')
+      screen:expect({
+        any = {
+          '{17:testing}',
+          '{17:m}%^ouse',
+          'support and selection',
+          'VISUAL',
+        },
+        unchanged = true,
+      })
+      feed('<LeftDrag><2,2>')
+      screen:expect({
+        any = {
+          '{17:testing}',
+          '{17:m}%^ouse',
+          'support and selection',
+          'VISUAL',
+        },
+        unchanged = true,
+      })
+      feed('<LeftRelease>')
+      screen:expect({
+        any = {
+          '{17:testing}',
+          '{17:m}%^ouse',
+          'support and selection',
+          'VISUAL',
+        },
+        unchanged = true,
+      })
+    end)
+
     it('left drag changes visual selection after tab click', function()
       feed_command('silent file foo | tabnew | file bar')
       insert('this is bar')