bug#20607: 24.5; Problem with overlays at same position but in different windows

[Eli Zaretskii]

> Date: Mon, 18 May 2015 22:14:20 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 20607@debbugs.gnu.org
> 
> > > Suppose I have two windows, WND1 and WND2, both displaying the same
> > > buffer. In WND1, there's an overlay from 1 to 2 with window property
> > > WND1 and display "ab". In WND2, there's an overlay from 1 to 3 with
> > > window property WND2 and display "abc".
> > > 
> > > Instead of the expected, I see "ab" in WND1, but "abcabc" in WND2.  When
> > > I `move-overlay' in the first window to 1-3, the glitch disappears, but
> > > I actually want the overlay to be 1-2, not 1-3. If I have to make it
> > > 1-3, I'd have to modify the display as well to include one more
> > > character from the buffer. I think it's better to fix the overlay logic
> > > rather than go for this work-around.
> > 
> > Please show the code to reproduce this.
> 
> Actually, you need not bother: I see it here.
> 
> For now, all I can say is that this is very old: Emacs 22 already
> behaves like that.  I think the problem is that the display engine
> bumps into the same overlay twice while walking the buffer, but I need
> to take a closer look to be sure.

Fixed in commit 18338b1, which also includes a fix to a related subtle
problem with cursor movement in this case (you couldn't put point at
position 2 in WND1 window: it would be "adjusted" to position 3).