bug#36269: bug#28620: Mouse drag event records wrong window for release when crossing frames

[martin rudalics <rudalics@gmx.at>]

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 > With Emacs 25.3 under MacOS 10.12, a drag with mouse-1 depressed from
 > the text area of frame F1 to the text area of frame F2 improperly
 > generates a drag event whose (posn-window (event-end <event>)) shows
 > F1 rather than F2.
 >
 > Note that for a drag between frames, posn-window should return a frame
 > (according to the Elisp manual but not its own doc string).  The bug is
 > that the event itself records the wrong frame (the depress frame rather
 > than the release frame).
 >
 > I have confirmed this with Emacs 25.2 under Windows 7 as well.

I tried to address this problem in the attached patch.  Tested with
GTK, Lucid, Motif and Windows builds.  Since my GNUstep Emacs is
currently broken, somebody please verify that it does something
reasonable (if anything at all) on MacOS.  Otherwise, I'd need help
from people working there.

The patch should also fix the mouse drag and drop region vs. mouse
avoidance mode problem.  Please someone verify that it does TRT now.

TIA, martin

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diff --git a/lisp/avoid.el b/lisp/avoid.el
index 7d69fa2a24..43e5062b76 100644
--- a/lisp/avoid.el
+++ b/lisp/avoid.el
@@ -327,6 +327,9 @@ mouse-avoidance-ignore-p
         executing-kbd-macro	       ; don't check inside macro
 	(null (cadr mp))	       ; don't move unless in an Emacs frame
 	(not (eq (car mp) (selected-frame)))
+        ;; Don't interfere with ongoing `mouse-drag-and-drop-region'
+        ;; (Bug#36269).
+        (eq track-mouse 'dropping)
 	;; Don't do anything if last event was a mouse event.
 	;; FIXME: this code fails in the case where the mouse was moved
 	;; since the last key-press but without generating any event.
diff --git a/lisp/mouse.el b/lisp/mouse.el
index 4a532a15e5..e947e16d47 100644
--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -1296,7 +1296,7 @@ mouse-drag-track
      t (lambda ()
          (setq track-mouse old-track-mouse)
          (setq auto-hscroll-mode auto-hscroll-mode-saved)
-          (deactivate-mark)
+         (deactivate-mark)
          (pop-mark)))))
 
 (defun mouse--drag-set-mark-and-point (start click click-count)
@@ -2467,12 +2467,13 @@ mouse-drag-and-drop-region
 
     (ignore-errors
       (track-mouse
+        (setq track-mouse 'dropping)
         ;; When event was "click" instead of "drag", skip loop.
         (while (progn
                  (setq event (read-key))      ; read-event or read-key
                  (or (mouse-movement-p event)
                      ;; Handle `mouse-autoselect-window'.
-                     (eq (car-safe event) 'select-window)))
+                     (memq (car event) '(select-window switch-frame))))
           ;; Obtain the dragged text in region.  When the loop was
           ;; skipped, value-selection remains nil.
           (unless value-selection
diff --git a/src/dispnew.c b/src/dispnew.c
index 0131b63767..74d53fb46d 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -3402,9 +3402,9 @@ update_window (struct window *w, bool force_p)
   if (!force_p)
     detect_input_pending_ignore_squeezables ();
 
-  /* If forced to complete the update, or if no input is pending, do
-     the update.  */
-  if (force_p || !input_pending || !NILP (do_mouse_tracking))
+  /* If forced to complete the update, no input is pending or we are
+     tracking the mouse do the update.  */
+  if (force_p || !input_pending || !NILP (track_mouse))
     {
       struct glyph_row *row, *end;
       struct glyph_row *mode_line_row;
diff --git a/src/keyboard.c b/src/keyboard.c
index b86ad03851..4224ab01b3 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1159,14 +1159,14 @@ DEFUN ("abort-recursive-edit", Fabort_recursive_edit, Sabort_recursive_edit, 0,
   user_error ("No recursive edit is in progress");
 }
 \f
-/* Restore mouse tracking enablement.  See Ftrack_mouse for the only use
-   of this function.  */
+/* Restore mouse tracking enablement.  See Finternal_track_mouse for
+   the only use of this function.  */
 
 static void
-tracking_off (Lisp_Object old_value)
+tracking_off (Lisp_Object old_track_mouse)
 {
-  do_mouse_tracking = old_value;
-  if (NILP (old_value))
+  track_mouse = old_track_mouse;
+  if (NILP (old_track_mouse))
     {
       /* Redisplay may have been preempted because there was input
 	 available, and it assumes it will be called again after the
@@ -1181,24 +1181,24 @@ tracking_off (Lisp_Object old_value)
     }
 }
 
-DEFUN ("internal--track-mouse", Ftrack_mouse, Strack_mouse, 1, 1, 0,
+DEFUN ("internal--track-mouse", Finternal_track_mouse, Sinternal_track_mouse,
+       1, 1, 0,
        doc: /* Call BODYFUN with mouse movement events enabled.  */)
   (Lisp_Object bodyfun)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
   Lisp_Object val;
 
-  record_unwind_protect (tracking_off, do_mouse_tracking);
+  record_unwind_protect (tracking_off, track_mouse);
 
-  do_mouse_tracking = Qt;
+  track_mouse = Qt;
 
   val = call0 (bodyfun);
   return unbind_to (count, val);
 }
 
-/* If mouse has moved on some frame, return one of those frames.
-
-   Return 0 otherwise.
+/* If mouse has moved on some frame and we are tracking the mouse,
+   return one of those frames.  Return NULL otherwise.
 
    If ignore_mouse_drag_p is non-zero, ignore (implicit) mouse movement
    after resizing the tool-bar window.  */
@@ -1210,11 +1210,8 @@ some_mouse_moved (void)
 {
   Lisp_Object tail, frame;
 
-  if (ignore_mouse_drag_p)
-    {
-      /* ignore_mouse_drag_p = false; */
-      return 0;
-    }
+  if (NILP (track_mouse) || ignore_mouse_drag_p)
+    return NULL;
 
   FOR_EACH_FRAME (tail, frame)
     {
@@ -1222,7 +1219,7 @@ some_mouse_moved (void)
 	return XFRAME (frame);
     }
 
-  return 0;
+  return NULL;
 }
 
 \f
@@ -2071,7 +2068,8 @@ show_help_echo (Lisp_Object help, Lisp_Object window, Lisp_Object object,
 	 This causes trouble if we are trying to read a mouse motion
 	 event (i.e., if we are inside a `track-mouse' form), so we
 	 restore the mouse_moved flag.  */
-      struct frame *f = NILP (do_mouse_tracking) ? NULL : some_mouse_moved ();
+      struct frame *f = some_mouse_moved ();
+
       help = call1 (Qmouse_fixup_help_message, help);
       if (f)
 	f->mouse_moved = true;
@@ -3403,8 +3401,7 @@ readable_events (int flags)
 	return 1;
     }
 
-  if (!(flags & READABLE_EVENTS_IGNORE_SQUEEZABLES)
-      && !NILP (do_mouse_tracking) && some_mouse_moved ())
+  if (!(flags & READABLE_EVENTS_IGNORE_SQUEEZABLES) && some_mouse_moved ())
     return 1;
   if (single_kboard)
     {
@@ -3786,7 +3783,7 @@ kbd_buffer_get_event (KBOARD **kbp,
 
       if (kbd_fetch_ptr != kbd_store_ptr)
 	break;
-      if (!NILP (do_mouse_tracking) && some_mouse_moved ())
+      if (some_mouse_moved ())
 	break;
 
       /* If the quit flag is set, then read_char will return
@@ -3802,7 +3799,7 @@ kbd_buffer_get_event (KBOARD **kbp,
 #endif
       if (kbd_fetch_ptr != kbd_store_ptr)
 	break;
-      if (!NILP (do_mouse_tracking) && some_mouse_moved ())
+      if (some_mouse_moved ())
 	break;
       if (end_time)
 	{
@@ -3941,8 +3938,9 @@ kbd_buffer_get_event (KBOARD **kbp,
         break;
       default:
 	{
-	  /* If this event is on a different frame, return a switch-frame this
-	     time, and leave the event in the queue for next time.  */
+	  /* If this event is on a different frame, return a
+	     switch-frame this time and leave the event in the queue
+	     for next time.  */
 	  Lisp_Object frame;
 	  Lisp_Object focus;
 
@@ -3956,14 +3954,13 @@ kbd_buffer_get_event (KBOARD **kbp,
 	  if (! NILP (focus))
 	    frame = focus;
 
-	  if (! EQ (frame, internal_last_event_frame)
+	  if (!EQ (frame, internal_last_event_frame)
 	      && !EQ (frame, selected_frame))
 	    obj = make_lispy_switch_frame (frame);
 	  internal_last_event_frame = frame;
 
 	  /* If we didn't decide to make a switch-frame event, go ahead
 	     and build a real event from the queue entry.  */
-
 	  if (NILP (obj))
 	    {
 	      obj = make_lispy_event (&event->ie);
@@ -3995,7 +3992,7 @@ kbd_buffer_get_event (KBOARD **kbp,
       }
     }
   /* Try generating a mouse motion event.  */
-  else if (!NILP (do_mouse_tracking) && some_mouse_moved ())
+  else if (some_mouse_moved ())
     {
       struct frame *f = some_mouse_moved ();
       Lisp_Object bar_window;
@@ -4027,7 +4024,7 @@ kbd_buffer_get_event (KBOARD **kbp,
 	  if (NILP (frame))
 	    XSETFRAME (frame, f);
 
-	  if (! EQ (frame, internal_last_event_frame)
+	  if (!EQ (frame, internal_last_event_frame)
 	      && !EQ (frame, selected_frame))
 	    obj = make_lispy_switch_frame (frame);
 	  internal_last_event_frame = frame;
@@ -10935,7 +10932,7 @@ init_keyboard (void)
   recent_keys_index = 0;
   kbd_fetch_ptr = kbd_buffer;
   kbd_store_ptr = kbd_buffer;
-  do_mouse_tracking = Qnil;
+  track_mouse = Qnil;
   input_pending = false;
   interrupt_input_blocked = 0;
   pending_signals = false;
@@ -11297,7 +11294,7 @@ syms_of_keyboard (void)
   defsubr (&Sread_key_sequence);
   defsubr (&Sread_key_sequence_vector);
   defsubr (&Srecursive_edit);
-  defsubr (&Strack_mouse);
+  defsubr (&Sinternal_track_mouse);
   defsubr (&Sinput_pending_p);
   defsubr (&Srecent_keys);
   defsubr (&Sthis_command_keys);
@@ -11642,8 +11639,15 @@ and the minor mode maps regardless of `overriding-local-map'.  */);
 	       doc: /* Keymap defining bindings for special events to execute at low level.  */);
   Vspecial_event_map = list1 (Qkeymap);
 
-  DEFVAR_LISP ("track-mouse", do_mouse_tracking,
-	       doc: /* Non-nil means generate motion events for mouse motion.  */);
+  DEFVAR_LISP ("track-mouse", track_mouse,
+	       doc: /* Non-nil means generate motion events for mouse motion.
+The sepecial values 'dragging' and 'dropping' assert that the moue
+cursor retains its appearance during mouse motion.  Any non-nil value
+but 'dropping' asserts that motion events always relate to the frame
+where the the mouse movement started.  The value 'dropping' asserts
+that motion events relate to the frame where the mouse cursor is seen
+when generating the event.  If there's no such frame, such motion
+events relate to the frame where the mouse movement started.  */);
 
   DEFVAR_KBOARD ("system-key-alist", Vsystem_key_alist,
 		 doc: /* Alist of system-specific X windows key symbols.
diff --git a/src/nsterm.m b/src/nsterm.m
index 02331826d9..75bf85617b 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -2480,7 +2480,11 @@ so some key presses (TAB) are swallowed by the system.  */
       XFRAME (frame)->mouse_moved = 0;
 
   dpyinfo->last_mouse_scroll_bar = nil;
+  f = dpyinfo->ns_focus_frame ? dpyinfo->ns_focus_frame : SELECTED_FRAME ();
   if (dpyinfo->last_mouse_frame
+      /* While dropping use the last mouse frame only if there is no
+	 currently focused frame.  */
+      && (!EQ (track_mouse, Qdropping) || !f)
       && FRAME_LIVE_P (dpyinfo->last_mouse_frame))
     f = dpyinfo->last_mouse_frame;
   else
diff --git a/src/term.c b/src/term.c
index b058d8bdad..a88d47f923 100644
--- a/src/term.c
+++ b/src/term.c
@@ -3033,18 +3033,18 @@ read_menu_input (struct frame *sf, int *x, int *y, int min_y, int max_y,
       bool usable_input = 1;
       mi_result st = MI_CONTINUE;
       struct tty_display_info *tty = FRAME_TTY (sf);
-      Lisp_Object saved_mouse_tracking = do_mouse_tracking;
+      Lisp_Object old_track_mouse = track_mouse;
 
       /* Signal the keyboard reading routines we are displaying a menu
 	 on this terminal.  */
       tty->showing_menu = 1;
       /* We want mouse movements be reported by read_menu_command.  */
-      do_mouse_tracking = Qt;
+      track_mouse = Qt;
       do {
 	cmd = read_menu_command ();
       } while (NILP (cmd));
       tty->showing_menu = 0;
-      do_mouse_tracking = saved_mouse_tracking;
+      track_mouse = old_track_mouse;
 
       if (EQ (cmd, Qt) || EQ (cmd, Qtty_menu_exit)
 	  /* If some input switched frames under our feet, exit the
diff --git a/src/w32fns.c b/src/w32fns.c
index acd9c80528..a2a88b2588 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -4586,7 +4586,8 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 	if (button_state & this)
 	  return 0;
 
-	if (button_state == 0)
+	/* Don't capture mouse when dropping.  */
+	if (button_state == 0 && !EQ (track_mouse, Qdropping))
 	  SetCapture (hwnd);
 
 	button_state |= this;
@@ -4707,8 +4708,11 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 
 	if (parse_button (msg, HIWORD (wParam), &button, &up))
 	  {
-	    if (up) ReleaseCapture ();
-	    else SetCapture (hwnd);
+	    if (up)
+	      ReleaseCapture ();
+	    /* Don't capture mouse when dropping.  */
+	    else if (!EQ (track_mouse, Qdropping))
+	      SetCapture (hwnd);
 	    button = (button == 0) ? LMOUSE :
 	      ((button == 1) ? MMOUSE  : RMOUSE);
 	    if (up)
@@ -5351,8 +5355,9 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 	else if (button_state & RMOUSE)
 	  flags |= TPM_RIGHTBUTTON;
 
-	/* Remember we did a SetCapture on the initial mouse down event,
-	   so for safety, we make sure the capture is canceled now.  */
+	/* We may have done a SetCapture on the initial mouse down
+	   event, so for safety, make sure the capture is canceled
+	   now.  */
 	ReleaseCapture ();
 	button_state = 0;
 
diff --git a/src/w32term.c b/src/w32term.c
index c6e175e7e5..ad96287a43 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -3525,72 +3525,78 @@ w32_mouse_position (struct frame **fp, int insist, Lisp_Object *bar_window,
 
       /* Now we have a position on the root; find the innermost window
 	 containing the pointer.  */
-      {
-	/* If mouse was grabbed on a frame, give coords for that
-	   frame even if the mouse is now outside it.  Otherwise
-	   check for window under mouse on one of our frames.  */
-	if (gui_mouse_grabbed (dpyinfo))
-	  f1 = dpyinfo->last_mouse_frame;
-	else
-	  {
-	    HWND wfp = WindowFromPoint (pt);
 
-	    if (wfp)
-	      {
-		f1 = w32_window_to_frame (dpyinfo, wfp);
-		if (f1)
-		  {
-		    HWND cwfp = ChildWindowFromPoint (wfp, pt);
+      /* If mouse was grabbed on a frame and we are not dropping,
+	 give coords for that frame even if the mouse is now outside
+	 it.  Otherwise check for window under mouse on one of our
+	 frames.  */
+      if (gui_mouse_grabbed (dpyinfo) && !EQ (track_mouse, Qdropping))
+	f1 = dpyinfo->last_mouse_frame;
+      else
+	{
+	  HWND wfp = WindowFromPoint (pt);
 
-		    if (cwfp)
-		      {
-			struct frame *f2 = w32_window_to_frame (dpyinfo, cwfp);
+	  if (wfp)
+	    {
+	      f1 = w32_window_to_frame (dpyinfo, wfp);
+	      if (f1)
+		{
+		  HWND cwfp = ChildWindowFromPoint (wfp, pt);
 
-			/* If a child window was found, make sure that its
-			   frame is a child frame (Bug#26615, maybe).  */
-			if (f2 && FRAME_PARENT_FRAME (f2))
-			  f1 = f2;
-		      }
-		  }
-	      }
-	  }
+		  if (cwfp)
+		    {
+		      struct frame *f2 = w32_window_to_frame (dpyinfo, cwfp);
 
-	/* If not, is it one of our scroll bars?  */
-	if (! f1)
-	  {
-	    struct scroll_bar *bar
-              = w32_window_to_scroll_bar (WindowFromPoint (pt), 2);
+		      /* If a child window was found, make sure that its
+			 frame is a child frame (Bug#26615, maybe).  */
+		      if (f2 && FRAME_PARENT_FRAME (f2))
+			f1 = f2;
+		    }
+		}
+	    }
+	}
 
-	    if (bar)
-	      f1 = XFRAME (WINDOW_FRAME (XWINDOW (bar->window)));
-	  }
+      if (!f1 || FRAME_TOOLTIP_P (f1))
+	/* Don't use a tooltip frame.  */
+	f1 = ((EQ (track_mouse, Qdropping) && gui_mouse_grabbed (dpyinfo))
+	      ? dpyinfo->last_mouse_frame
+	      : NULL);
 
-	if (f1 == 0 && insist > 0)
-	  f1 = SELECTED_FRAME ();
+      /* If not, is it one of our scroll bars?  */
+      if (!f1)
+	{
+	  struct scroll_bar *bar
+	    = w32_window_to_scroll_bar (WindowFromPoint (pt), 2);
 
-	if (f1)
-	  {
-	    /* Ok, we found a frame.  Store all the values.
-	       last_mouse_glyph is a rectangle used to reduce the
-	       generation of mouse events.  To not miss any motion
-	       events, we must divide the frame into rectangles of the
-	       size of the smallest character that could be displayed
-	       on it, i.e. into the same rectangles that matrices on
-	       the frame are divided into.  */
-
-	    dpyinfo = FRAME_DISPLAY_INFO (f1);
-	    ScreenToClient (FRAME_W32_WINDOW (f1), &pt);
-	    remember_mouse_glyph (f1, pt.x, pt.y, &dpyinfo->last_mouse_glyph);
-	    dpyinfo->last_mouse_glyph_frame = f1;
-
-	    *bar_window = Qnil;
-	    *part = scroll_bar_above_handle;
-	    *fp = f1;
-	    XSETINT (*x, pt.x);
-	    XSETINT (*y, pt.y);
-	    *time = dpyinfo->last_mouse_movement_time;
-	  }
-      }
+	  if (bar)
+	    f1 = XFRAME (WINDOW_FRAME (XWINDOW (bar->window)));
+	}
+
+      if (!f1 && insist > 0)
+	f1 = SELECTED_FRAME ();
+
+      if (f1)
+	{
+	  /* Ok, we found a frame.  Store all the values.
+	     last_mouse_glyph is a rectangle used to reduce the
+	     generation of mouse events.  To not miss any motion
+	     events, we must divide the frame into rectangles of the
+	     size of the smallest character that could be displayed
+	     on it, i.e. into the same rectangles that matrices on
+	     the frame are divided into.  */
+
+	  dpyinfo = FRAME_DISPLAY_INFO (f1);
+	  ScreenToClient (FRAME_W32_WINDOW (f1), &pt);
+	  remember_mouse_glyph (f1, pt.x, pt.y, &dpyinfo->last_mouse_glyph);
+	  dpyinfo->last_mouse_glyph_frame = f1;
+
+	  *bar_window = Qnil;
+	  *part = scroll_bar_above_handle;
+	  *fp = f1;
+	  XSETINT (*x, pt.x);
+	  XSETINT (*y, pt.y);
+	  *time = dpyinfo->last_mouse_movement_time;
+	}
     }
 
   unblock_input ();
@@ -4667,6 +4673,37 @@ static short temp_buffer[100];
 /* Temporarily store lead byte of DBCS input sequences.  */
 static char dbcs_lead = 0;
 
+/**
+  mouse_or_wdesc_frame: When not dropping and the mouse was grabbed
+  for DPYINFO, return the frame where the mouse was seen last.  If
+  there's no such frame, return the frame according to WDESC.  When
+  dropping, return the frame according to WDESC.  If there's no such
+  frame and the mouse was grabbed for DPYINFO, return the frame where
+  the mouse was seen last.  In either case, never return a tooltip
+  frame.  */
+static struct frame *
+mouse_or_wdesc_frame (struct w32_display_info *dpyinfo, HWND wdesc)
+{
+  struct frame *lm_f = (gui_mouse_grabbed (dpyinfo)
+			? dpyinfo->last_mouse_frame
+			: NULL);
+
+  if (lm_f && !EQ (track_mouse, Qdropping))
+    return lm_f;
+  else
+    {
+      struct frame *w_f = w32_window_to_frame (dpyinfo, wdesc);
+
+      /* Do not return a tooltip frame.  */
+      if (!w_f || FRAME_TOOLTIP_P (w_f))
+	return EQ (track_mouse, Qdropping) ? lm_f : NULL;
+      else
+	/* When dropping it would be probably nice to raise w_f
+	   here.  */
+	return w_f;
+    }
+}
+
 /* Read events coming from the W32 shell.
    This routine is called by the SIGIO handler.
    We return as soon as there are no more events to be read.
@@ -4940,15 +4977,13 @@ w32_read_socket (struct terminal *terminal,
           previous_help_echo_string = help_echo_string;
 	  help_echo_string = Qnil;
 
-	  f = (gui_mouse_grabbed (dpyinfo) ? dpyinfo->last_mouse_frame
-	       : w32_window_to_frame (dpyinfo, msg.msg.hwnd));
-
 	  if (hlinfo->mouse_face_hidden)
 	    {
 	      hlinfo->mouse_face_hidden = false;
 	      clear_mouse_face (hlinfo);
 	    }
 
+	  f = mouse_or_wdesc_frame (dpyinfo, msg.msg.hwnd);
 	  if (f)
 	    {
 	      /* Maybe generate SELECT_WINDOW_EVENTs for
@@ -5020,9 +5055,7 @@ w32_read_socket (struct terminal *terminal,
 	    int button = 0;
 	    int up = 0;
 
-	    f = (gui_mouse_grabbed (dpyinfo) ? dpyinfo->last_mouse_frame
-		 : w32_window_to_frame (dpyinfo, msg.msg.hwnd));
-
+	    f = mouse_or_wdesc_frame (dpyinfo, msg.msg.hwnd);
 	    if (f)
 	      {
                 w32_construct_mouse_click (&inev, &msg, f);
@@ -5081,9 +5114,7 @@ w32_read_socket (struct terminal *terminal,
 	case WM_MOUSEWHEEL:
         case WM_MOUSEHWHEEL:
 	  {
-	    f = (gui_mouse_grabbed (dpyinfo) ? dpyinfo->last_mouse_frame
-		 : w32_window_to_frame (dpyinfo, msg.msg.hwnd));
-
+	    f = mouse_or_wdesc_frame (dpyinfo, msg.msg.hwnd);
 	    if (f)
 	      {
 		if (!dpyinfo->w32_focus_frame
@@ -5439,6 +5470,7 @@ w32_read_socket (struct terminal *terminal,
 	      if (any_help_event_p)
 		do_help = -1;
 	    }
+
 	  break;
 
 	case WM_SETFOCUS:
diff --git a/src/xdisp.c b/src/xdisp.c
index 1bb5f5e0f2..7338d2b7d4 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -17289,7 +17289,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
 	 the mouse, resulting in an unwanted mouse-movement rather
 	 than a simple mouse-click.  */
       if (!w->start_at_line_beg
-	  && NILP (do_mouse_tracking)
+	  && NILP (track_mouse)
       	  && CHARPOS (startp) > BEGV
 	  && CHARPOS (startp) > BEG + beg_unchanged
 	  && CHARPOS (startp) <= Z - end_unchanged
@@ -30279,7 +30279,7 @@ show_mouse_face (Mouse_HLInfo *hlinfo, enum draw_glyphs_face draw)
 
 #ifdef HAVE_WINDOW_SYSTEM
   /* Change the mouse cursor.  */
-  if (FRAME_WINDOW_P (f) && NILP (do_mouse_tracking))
+  if (FRAME_WINDOW_P (f) && NILP (track_mouse))
     {
 #ifndef HAVE_EXT_TOOL_BAR
       if (draw == DRAW_NORMAL_TEXT
@@ -31226,7 +31226,7 @@ define_frame_cursor1 (struct frame *f, Emacs_Cursor cursor, Lisp_Object pointer)
     return;
 
   /* Do not change cursor shape while dragging mouse.  */
-  if (EQ (do_mouse_tracking, Qdragging))
+  if (EQ (track_mouse, Qdragging) || EQ (track_mouse, Qdropping))
     return;
 
   if (!NILP (pointer))
@@ -32956,6 +32956,7 @@ be let-bound around code that needs to disable messages temporarily. */);
   /* also Qtext */
 
   DEFSYM (Qdragging, "dragging");
+  DEFSYM (Qdropping, "dropping");
 
   DEFSYM (Qinhibit_free_realized_faces, "inhibit-free-realized-faces");
 
diff --git a/src/xterm.c b/src/xterm.c
index c96aa74a7a..97d14f7d45 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -5167,20 +5167,15 @@ XTmouse_position (struct frame **fp, int insist, Lisp_Object *bar_window,
       /* Figure out which root window we're on.  */
       XQueryPointer (FRAME_X_DISPLAY (*fp),
 		     DefaultRootWindow (FRAME_X_DISPLAY (*fp)),
-
 		     /* The root window which contains the pointer.  */
 		     &root,
-
 		     /* Trash which we can't trust if the pointer is on
 			a different screen.  */
 		     &dummy_window,
-
 		     /* The position on that root window.  */
 		     &root_x, &root_y,
-
 		     /* More trash we can't trust.  */
 		     &dummy, &dummy,
-
 		     /* Modifier keys and pointer buttons, about which
 			we don't care.  */
 		     (unsigned int *) &dummy);
@@ -5203,21 +5198,17 @@ XTmouse_position (struct frame **fp, int insist, Lisp_Object *bar_window,
 
 	x_catch_errors (FRAME_X_DISPLAY (*fp));
 
-	if (gui_mouse_grabbed (dpyinfo))
+	if (gui_mouse_grabbed (dpyinfo) && !EQ (track_mouse, Qdropping))
 	  {
 	    /* If mouse was grabbed on a frame, give coords for that frame
 	       even if the mouse is now outside it.  */
 	    XTranslateCoordinates (FRAME_X_DISPLAY (*fp),
-
 				   /* From-window.  */
 				   root,
-
 				   /* To-window.  */
 				   FRAME_X_WINDOW (dpyinfo->last_mouse_frame),
-
 				   /* From-position, to-position.  */
 				   root_x, root_y, &win_x, &win_y,
-
 				   /* Child of win.  */
 				   &child);
 	    f1 = dpyinfo->last_mouse_frame;
@@ -5227,16 +5218,12 @@ XTmouse_position (struct frame **fp, int insist, Lisp_Object *bar_window,
 	    while (true)
 	      {
 		XTranslateCoordinates (FRAME_X_DISPLAY (*fp),
-
 				       /* From-window, to-window.  */
 				       root, win,
-
 				       /* From-position, to-position.  */
 				       root_x, root_y, &win_x, &win_y,
-
 				       /* Child of win.  */
 				       &child);
-
 		if (child == None || child == win)
 		  {
 #ifdef USE_GTK
@@ -5299,13 +5286,35 @@ XTmouse_position (struct frame **fp, int insist, Lisp_Object *bar_window,
 #endif /* USE_X_TOOLKIT */
 	  }
 
+	if ((!f1 || FRAME_TOOLTIP_P (f1))
+	    && EQ (track_mouse, Qdropping)
+	    && gui_mouse_grabbed (dpyinfo))
+	  {
+	    /* When dropping then if we didn't get a frame or only a
+	       tooltip frame and the mouse was grabbed on a frame,
+	       give coords for that frame even if the mouse is now
+	       outside it.  */
+	    XTranslateCoordinates (FRAME_X_DISPLAY (*fp),
+				   /* From-window.  */
+				   root,
+				   /* To-window.  */
+				   FRAME_X_WINDOW (dpyinfo->last_mouse_frame),
+				   /* From-position, to-position.  */
+				   root_x, root_y, &win_x, &win_y,
+				   /* Child of win.  */
+				   &child);
+	    f1 = dpyinfo->last_mouse_frame;
+	  }
+	else if (f1 && FRAME_TOOLTIP_P (f1))
+	  f1 = NULL;
+
 	if (x_had_errors_p (FRAME_X_DISPLAY (*fp)))
-	  f1 = 0;
+	  f1 = NULL;
 
 	x_uncatch_errors_after_check ();
 
 	/* If not, is it one of our scroll bars?  */
-	if (! f1)
+	if (!f1)
 	  {
 	    struct scroll_bar *bar;
 
@@ -5319,7 +5328,7 @@ XTmouse_position (struct frame **fp, int insist, Lisp_Object *bar_window,
 	      }
 	  }
 
-	if (f1 == 0 && insist > 0)
+	if (!f1 && insist > 0)
 	  f1 = SELECTED_FRAME ();
 
 	if (f1)
@@ -7788,6 +7797,37 @@ flush_dirty_back_buffers (void)
   unblock_input ();
 }
 
+/**
+  mouse_or_wdesc_frame: When not dropping and the mouse was grabbed
+  for DPYINFO, return the frame where the mouse was seen last.  If
+  there's no such frame, return the frame according to WDESC.  When
+  dropping, return the frame according to WDESC.  If there's no such
+  frame and the mouse was grabbed for DPYINFO, return the frame where
+  the mouse was seen last.  In either case, never return a tooltip
+  frame.  */
+static struct frame *
+mouse_or_wdesc_frame (struct x_display_info *dpyinfo, int wdesc)
+{
+  struct frame *lm_f = (gui_mouse_grabbed (dpyinfo)
+			? dpyinfo->last_mouse_frame
+			: NULL);
+
+  if (lm_f && !EQ (track_mouse, Qdropping))
+    return lm_f;
+  else
+    {
+      struct frame *w_f = x_window_to_frame (dpyinfo, wdesc);
+
+      /* Do not return a tooltip frame.  */
+      if (!w_f || FRAME_TOOLTIP_P (w_f))
+	return EQ (track_mouse, Qdropping) ? lm_f : NULL;
+      else
+	/* When dropping it would be probably nice to raise w_f
+	   here.  */
+	return w_f;
+    }
+}
+
 /* Handles the XEvent EVENT on display DPYINFO.
 
    *FINISH is X_EVENT_GOTO_OUT if caller should stop reading events.
@@ -8720,15 +8760,14 @@ handle_one_xevent (struct x_display_info *dpyinfo,
         previous_help_echo_string = help_echo_string;
         help_echo_string = Qnil;
 
-	f = (gui_mouse_grabbed (dpyinfo) ? dpyinfo->last_mouse_frame
-	     : x_window_to_frame (dpyinfo, event->xmotion.window));
-
-        if (hlinfo->mouse_face_hidden)
+	if (hlinfo->mouse_face_hidden)
           {
             hlinfo->mouse_face_hidden = false;
             clear_mouse_face (hlinfo);
           }
 
+	f = mouse_or_wdesc_frame (dpyinfo, event->xmotion.window);
+
 #ifdef USE_GTK
         if (f && xg_event_is_for_scrollbar (f, event))
           f = 0;
@@ -8970,33 +9009,27 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 	dpyinfo->last_mouse_glyph_frame = NULL;
 	x_display_set_last_user_time (dpyinfo, event->xbutton.time);
 
-	if (gui_mouse_grabbed (dpyinfo))
-	  f = dpyinfo->last_mouse_frame;
-	else
+	f = mouse_or_wdesc_frame (dpyinfo, event->xmotion.window);
+	if (f && event->xbutton.type == ButtonPress
+	    && !popup_activated ()
+	    && !x_window_to_scroll_bar (event->xbutton.display,
+					event->xbutton.window, 2)
+	    && !FRAME_NO_ACCEPT_FOCUS (f))
 	  {
-	    f = x_window_to_frame (dpyinfo, event->xbutton.window);
+	    /* When clicking into a child frame or when clicking
+	       into a parent frame with the child frame selected and
+	       `no-accept-focus' is not set, select the clicked
+	       frame.  */
+	    struct frame *hf = dpyinfo->highlight_frame;
 
-	    if (f && event->xbutton.type == ButtonPress
-		&& !popup_activated ()
-		&& !x_window_to_scroll_bar (event->xbutton.display,
-					    event->xbutton.window, 2)
-		&& !FRAME_NO_ACCEPT_FOCUS (f))
+	    if (FRAME_PARENT_FRAME (f) || (hf && frame_ancestor_p (f, hf)))
 	      {
-		/* When clicking into a child frame or when clicking
-		   into a parent frame with the child frame selected and
-		   `no-accept-focus' is not set, select the clicked
-		   frame.  */
-		struct frame *hf = dpyinfo->highlight_frame;
-
-		if (FRAME_PARENT_FRAME (f) || (hf && frame_ancestor_p (f, hf)))
-		  {
-		    block_input ();
-		    XSetInputFocus (FRAME_X_DISPLAY (f), FRAME_OUTER_WINDOW (f),
-				    RevertToParent, CurrentTime);
-		    if (FRAME_PARENT_FRAME (f))
-		      XRaiseWindow (FRAME_X_DISPLAY (f), FRAME_OUTER_WINDOW (f));
-		    unblock_input ();
-		  }
+		block_input ();
+		XSetInputFocus (FRAME_X_DISPLAY (f), FRAME_OUTER_WINDOW (f),
+				RevertToParent, CurrentTime);
+		if (FRAME_PARENT_FRAME (f))
+		  XRaiseWindow (FRAME_X_DISPLAY (f), FRAME_OUTER_WINDOW (f));
+		unblock_input ();
 	      }
 	  }