bug#14744: 24.3.50; Flickering mouse-face on process output

[Eli Zaretskii <eliz@gnu.org>]

> From: Christopher Schmidt <christopher@ch.ristopher.com>
> Date: Sat, 29 Jun 2013 13:56:53 +0100 (BST)
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> > For the record, what is the real-life use case where this matters?
> 
> The one Stephen described in <8761wxfbm8.fsf@rosalinde.fritz.box>.

Thanks.  For the record, here's what happens:

  . comint-output-filter moves the comint-last-prompt-overlay overlay

  . This eventually calls modify_overlay, which has this code:

     /* If BUF is visible, consider updating the display if ...  */
     if (buffer_window_count (buf) > 0)
       {
	 /* ... it's visible in other window than selected,  */
	 if (buf != XBUFFER (XWINDOW (selected_window)->contents))
	   windows_or_buffers_changed = 1;
	 /* ... or if we modify an overlay at the end of the buffer
	    and so we cannot be sure that window end is still valid.  */
	 else if (end >= ZV && start <= ZV)
	   windows_or_buffers_changed = 1;
       }

    In our case, the overlay is at the end of the buffer, so the last
    'else if' clause fires, and sets windows_or_buffers_changed to a
    non-zero value.

  . When redisplay sees a non-zero value in windows_or_buffers_changed,
    it forces a thorough redisplay of all the windows, because having
    the window end invalid generally means the window configuration
    might have changed.

I guess one way of fixing this problem would be to modify comint.el
not to use overlays for this purpose.