bug#50660: 28.0.50; Text artifacting when the cursor moves over text under mouse face that originally displayed a box

[Eli Zaretskii]

> Date: Tue, 21 Sep 2021 13:17:58 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: larsi@gnus.org, 50660@debbugs.gnu.org
> 
> > From: Po Lu <luangruo@yahoo.com>
> > Cc: larsi@gnus.org,  50660@debbugs.gnu.org
> > Date: Tue, 21 Sep 2021 17:45:37 +0800
> > 
> > So even if I move the cursor into the middle of the box, it will
> > still be in the wrong location, but when the cursor is moved,
> > show_mouse_face is not called, so I think putting the correction is
> > show_mouse_face is not the solution to this problem.
> 
> We could identify this situation and call show_mouse_face forcibly,
> e.g. in gui_update_window_end or some such place.

Here's a specific idea: add the same code which fixes the cursor
position into gui_update_window_end, before display_and_set_cursor is
called, here:

  if (!w->pseudo_window_p)
    {
      block_input ();

      if (cursor_on_p)
        display_and_set_cursor (w, true,
                                w->output_cursor.hpos, w->output_cursor.vpos,
                                w->output_cursor.x, w->output_cursor.y);

      if (draw_window_fringes (w, true))
        {
          if (WINDOW_RIGHT_DIVIDER_WIDTH (w))
            gui_draw_right_divider (w);
          else
            gui_draw_vertical_border (w);
        }
      unblock_input ();
    }

This is always called when the display is updated, even if we just
move the cursor.  By looking at the value of cursor_in_mouse_face_p,
we can determine whether the fix is needed, and apply it before
calling display_and_set_cursor.  (We will need to fix w->phys_cursor.x
after the call to display_and_set_cursor returns, because it records
there the value passed as the 5th argument.)