emacs-28 04716ca: Add tab-bar-drag-maybe for indication of tab dragging (bug#50993)

[Juri Linkov]

branch: emacs-28
commit 04716ca48fbaf4750a56e3c8d6f0ad125f889105
Author: Martin Rudalics <rudalics@gmx.at>
Commit: Juri Linkov <juri@linkov.net>

    Add tab-bar-drag-maybe for indication of tab dragging (bug#50993)
    
    * lisp/tab-bar.el (tab-bar--event-to-item)
    (tab-bar-mouse-move-tab): Set tab-bar-drag-maybe to nil.
    (tab-bar-mouse-down-1): Set tab-bar-drag-maybe to t.
    
    * src/xdisp.c (note_mouse_highlight): Set cursor to 'hand_cursor'
    when tab_bar_drag_maybe is true.
    (syms_of_xdisp): New variable tab-bar-drag-maybe.
---
 lisp/tab-bar.el |  5 +++++
 src/xdisp.c     | 12 +++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/lisp/tab-bar.el b/lisp/tab-bar.el
index 9fb363d..76a217c 100644
--- a/lisp/tab-bar.el
+++ b/lisp/tab-bar.el
@@ -239,11 +239,14 @@ and `t' for other values."
         (string-to-number (string-replace "tab-" "" key-name)))))
    (t t)))
 
+(defvar tab-bar-drag-maybe)
+
 (defun tab-bar--event-to-item (posn)
   "This function extracts extra info from the mouse event POSN.
 It returns a list that contains three elements: a key,
 a key binding, and a boolean value whether the close button \"+\"
 was clicked."
+  (setq tab-bar-drag-maybe nil)
   (if (posn-window posn)
       (let ((caption (car (posn-string posn))))
         (when caption
@@ -275,6 +278,7 @@ existing tab."
   (interactive "e")
   (let* ((item (tab-bar--event-to-item (event-start event)))
          (tab-number (tab-bar--key-to-number (nth 0 item))))
+    (setq tab-bar-drag-maybe t)
     ;; Don't close the tab when clicked on the close button.  Also
     ;; don't add new tab on down-mouse.  Let `tab-bar-mouse-1' do this.
     (unless (or (eq (car item) 'add-tab) (nth 2 item))
@@ -351,6 +355,7 @@ only when you click on its \"x\" close button."
 This command should be bound to a drag event.  It moves the tab
 at the mouse-down event to the position at mouse-up event."
   (interactive "e")
+  (setq tab-bar-drag-maybe nil)
   (let ((from (tab-bar--key-to-number
                (nth 0 (tab-bar--event-to-item
                        (event-start event)))))
diff --git a/src/xdisp.c b/src/xdisp.c
index 40d578a..dc92725 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -33644,7 +33644,13 @@ note_mouse_highlight (struct frame *f, int x, int y)
   if (EQ (window, f->tab_bar_window))
     {
       note_tab_bar_highlight (f, x, y);
-      return;
+      if (tab_bar_drag_maybe)
+       {
+         cursor = FRAME_OUTPUT_DATA (f)->hand_cursor;
+         goto set_cursor;
+       }
+      else
+       return;
     }
 #endif
 
@@ -35774,6 +35780,10 @@ When nil, mouse-movement events will not be generated 
as long as the
 mouse stays within the extent of a single glyph (except for images).  */);
   mouse_fine_grained_tracking = false;
 
+  DEFVAR_BOOL ("tab-bar-drag-maybe", tab_bar_drag_maybe,
+    doc: /* Non-nil when maybe dragging tab bar item.  */);
+  tab_bar_drag_maybe = false;
+
   DEFVAR_BOOL ("redisplay-skip-initial-frame", redisplay_skip_initial_frame,
     doc: /* Non-nil to skip redisplay in initial frame.
 The initial frame is not displayed anywhere, so skipping it is