bug#7464: 24.0.50; mouse highlighting vanishes upon unsplitting window

[Stephen Berman]

On Wed, 28 Mar 2012 20:56:14 +0200 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: cyd@gnu.org,  7464@debbugs.gnu.org
>> Date: Sun, 25 Mar 2012 14:57:18 +0200
>> 
>> >> why do I not see the bug in Emacs 23?  (And why does Chong Yidong
>> >> not see it in Emacs 24?)
>> >
>> > I don't know.  I can describe in detail what I saw in the debugger and
>> > where, and then you and Chong could step through the same places and
>> > see what, if anything, is different for you there.
>> 
>> It would interest me to try, when you have the time.
>
> Here's what I see in GDB when I step through Emacs 24 code
> (delete-other-windows-internal):

Thanks for the very instructive debugging details; they led to some
unexpected observations, which I hope you can (help me) make sense of.

Since unlike you I did't see the highlighting disappear in Emacs 23, I
wanted to try and follow your instructions there, and since I don't have
the C sources of 23.3 from openSUSE installed on my machine, I
downloaded the tarball of 23.4, configured and built it, and tested the
bug recipe -- and to my surprise, I hit the bug: the mouse highlighting
vanished, just as I observe with Emacs 24.  I don't think the window
deletion code of 23.4 differs from that of 23.4, but the two builds do
differ in a way I didn't think would be relevant, but proved to be:
openSUSE's 23.3 has GTK scroll bars, while I built 23.4 like I build 24,
i.e., without toolkit scroll bars.  So I rebuilt 23.4 with GTK scroll
bars, did the bug recipe and sure enough, now the mouse highlighting
remained, i.e., no bug.  Then I disabled the scroll bars in that build,
tested again, and got the bug again.  Then I rebuilt Emacs 24 with GTK
scroll bars, and now did not see the bug with them enabled but did see
it with them disabled.  So the answers to my questions above appear to
involve the presence vs. absence of GTK scroll bars.  (I guess Chong
Yidong also builds Emacs 24 with GTK scroll bars and tested the recipe
with them enabled, so that's why he did not observe the bug.)

After these tests, I ran both Emacs 24 builds (i.e., with and without
GTK scroll bars) under gdb, and in both cases, the value of
(w->current_matrix->rows+1)->mouse_face_p is 0 both at the breakpoint
and after the call to free_window_matrices (and of course after
adjust_glyphs).  I also ran 23.4 under gdb, with the same result:
(w->current_matrix->rows+1)->mouse_face_p is already 0 before
adjust_glyphs.  Do you have any idea why, or any further debugging
suggestions?

Steve Berman