bug#10903: 24.0.93; redisplay glitch with before-string and display overlays

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Berman <stephen.berman@gmx.net>
> Date: Mon, 27 Feb 2012 23:04:03 +0100
> 
> 1. emacs -Q
> 
> 2. Type `C-x b a RET' to get a fresh buffer in Fundamental mode, and  at
> (point-min) type "012".
> 
> 3. Type `M-<' so that the cursor is over (i.e. just before) `0'.
> 
> 4. Type `M-: (overlay-put (make-overlay (point) (point)) 'before-string
> "*")'.  Now you see this:
> 
> *012
> 
> 5. Type `M-: (overlay-put (make-overlay (point) (1+ (point))) 'display
> "")'.  Now you see this:
> 
> *12
> 
> 6. Type `M-: (remove-overlays)' to restore "012".
> 
> 7. Type SPC to advance the cursor and the start of "012" to column one.
> 
> 8. Repeat step 4.  Now you see this:
> 
>  *012
> 
> 9. Repeat step 5.  Now you see this:
> 
>  *112

We were hitting on an old redisplay optimization that didn't play well
with the bidi-aware display engine.

I think I fixed this.  Thanks for such a clear-cut test case.

Since Savannah is on the move, I cannot commit the fix, but perhaps
you could test it locally and see if there are any leftovers.  Here's
the patch:

=== modified file 'src/xdisp.c'
--- src/xdisp.c	2012-02-12 09:46:33 +0000
+++ src/xdisp.c	2012-02-28 20:35:41 +0000
@@ -5156,6 +5156,10 @@ next_overlay_string (struct it *it)
       it->current.overlay_string_index = -1;
       it->n_overlay_strings = 0;
       it->overlay_strings_charpos = -1;
+      /* If there's an empty display string on the stack, pop the
+	 stack, to resync the bidi iterator with IT's position.  */
+      if (it->sp > 0 && STRINGP (it->string) && !SCHARS (it->string))
+	pop_it (it);
 
       /* If we're at the end of the buffer, record that we have
 	 processed the overlay strings there already, so that
@@ -5453,8 +5457,15 @@ get_overlay_strings_1 (struct it *it, EM
       xassert (!compute_stop_p || it->sp == 0);
 
       /* When called from handle_stop, there might be an empty display
-         string loaded.  In that case, don't bother saving it.  */
-      if (!STRINGP (it->string) || SCHARS (it->string))
+         string loaded.  In that case, don't bother saving it.  But
+         don't use this optimization with the bidi iterator, since we
+         need the corresponding pop_it call to resync the bidi
+         iterator's position with IT's position, after we are done
+         with the overlay strings.  (The corresponding call to pop_it
+         in case of an empty display string is in
+         next_overlay_string.)  */
+      if (!(!it->bidi_p
+	    && STRINGP (it->string) && !SCHARS (it->string)))
 	push_it (it, NULL);
 
       /* Set up IT to deliver display elements from the first overlay