Package: emacs;
Reported by: Markus Triska <triska <at> metalevel.at>
Date: Mon, 10 Feb 2025 21:57:01 UTC
Severity: normal
Found in version 31.0.50
Message #59 received at 76186 <at> debbugs.gnu.org (full text, mbox):
From: Stephen Berman <stephen.berman <at> gmx.net> To: Eli Zaretskii <eliz <at> gnu.org> Cc: 76186 <at> debbugs.gnu.org, triska <at> metalevel.at Subject: Re: bug#76186: 31.0.50; (recenter 0) sometimes does not recenter as expected Date: Tue, 25 Feb 2025 16:48:55 +0100
On Sun, 23 Feb 2025 09:02:35 +0200 Eli Zaretskii <eliz <at> gnu.org> wrote:
>> From: Stephen Berman <stephen.berman <at> gmx.net>
>> Cc: triska <at> metalevel.at, 76186 <at> debbugs.gnu.org
>> Date: Sat, 22 Feb 2025 22:06:18 +0100
>>
>> As you can see, after hitting the window.c breakpoint and stepping over
>> set_marker_both, window-start is at 793, i.e. point-max, which is the
>> expected position after invoking `(recenter 0)'. After continuing and
>> hitting the xdisp.c breakpoint, I stepped through the code and saw
>> w->force_start get set to true, then stepped further until
>> redisplay_window_0, then continued and hit the breakpoint again, and now
>> w->force_start was false. Then I stepped further until
>> SET_TEXT_POS_FROM_MARKER (startp, w->start), after which window-start
>> was now at 671, the value displayed in the message after one iteration,
>> indicating the unexpected recentering. I cannot tell what caused
>> window-start to change from 793 to 671.
>
> Thanks.
>
> There are too many unknowns here that need to become knowns.
>
> First, before the first call to redisplay_window returns, it does
> this:
>
> /* Restore current_buffer and value of point in it. The window
> update may have changed the buffer, so first make sure `opoint'
> is still valid (Bug#6177). */
> if (CHARPOS (opoint) < BEGV)
> TEMP_SET_PT_BOTH (BEGV, BEGV_BYTE);
> else if (CHARPOS (opoint) > ZV)
> TEMP_SET_PT_BOTH (Z, Z_BYTE);
> else if (ochars_modiff == CHARS_MODIFF)
> TEMP_SET_PT_BOTH (CHARPOS (opoint), BYTEPOS (opoint));
> else
> {
> /* If the buffer was modified while we were redisplaying it, we
> cannot trust the correspondence between character and byte
> positions. This can happen, for example, if we are
> redisplaying *Messages* and some Lisp, perhaps invoked by
> display_mode_lines, signals an error which caused something
> added/deleted to/from the buffer text. */
> TEMP_SET_PT_BOTH (CHARPOS (opoint), CHAR_TO_BYTE (CHARPOS (opoint)));
> }
> set_buffer_internal_1 (old);
> /* Avoid an abort in TEMP_SET_PT_BOTH if the buffer has become
> shorter. This can be caused by log truncation in *Messages*. */
> if (CHARPOS (lpoint) <= ZV)
> {
> if (lchars_modiff == CHARS_MODIFF)
> TEMP_SET_PT_BOTH (CHARPOS (lpoint), BYTEPOS (lpoint));
> else
> TEMP_SET_PT_BOTH (CHARPOS (lpoint), CHAR_TO_BYTE (CHARPOS (lpoint)));
> }
>
> Your trace indicates that TEMP_SET_PT_BOTH was called twice when
> executing this fragment, but you haven't shown the values of point
> after each one of them. It is crucial to understand where point was
> after the first call to redisplay_window, because any subsequent
> scrolling of the window depends on whether point is fully visible in
> the viewport. So please show the value of point as set by these two
> calls to TEMP_SET_PT_BOTH.
Thread 1 "emacs" hit Breakpoint 3, redisplay_window (window=XIL(0x555555c826c5),
just_this_one_p=just_this_one_p <at> entry=false)
at /home/steve/src/emacs/emacs-master/src/xdisp.c:21195
21195 if (CHARPOS (opoint) < BEGV)
(gdb) p w->start
$1 = XIL(0x555555d2782d)
(gdb) pr $1
#<marker at 793 in smple>
(gdb) p PT
$2 = 793
(gdb) p ZV
$3 = 793
(gdb) n
[Thread 0x7fffde16e6c0 (LWP 28706) exited]
21197 else if (CHARPOS (opoint) > ZV)
(gdb)
21199 else if (ochars_modiff == CHARS_MODIFF)
(gdb)
21200 TEMP_SET_PT_BOTH (CHARPOS (opoint), BYTEPOS (opoint));
(gdb)
21211 set_buffer_internal_1 (old);
(gdb) p PT
$4 = 793
(gdb) n
21214 if (CHARPOS (lpoint) <= ZV)
(gdb)
21216 if (lchars_modiff == CHARS_MODIFF)
(gdb)
21217 TEMP_SET_PT_BOTH (CHARPOS (lpoint), BYTEPOS (lpoint));
(gdb)
21222 unbind_to (count, Qnil);
(gdb) p PT
$5 = 793
> Next, at the very beginning of redisplay_window we have this:
>
> if (!just_this_one_p && needs_no_redisplay (w))
> return;
>
> Thus, the second call to redisplay_window was supposed to return
> immediately if these conditions were true. Since it didn't, and
> just_this_one_p is false, we can conclude that needs_no_redisplay
> returned false, but which condition inside it caused that?
(gdb) break xdisp.c:20081
Breakpoint 4 at 0x5555555f8ae8: file /home/steve/src/emacs/emacs-master/src/xdisp.c, line 20081.
(gdb) c
Continuing.
Thread 1 "emacs" hit Breakpoint 4, redisplay_window (window=XIL(0x555555c828e5),
just_this_one_p=just_this_one_p <at> entry=false)
at /home/steve/src/emacs/emacs-master/src/xdisp.c:20081
20081 if (!just_this_one_p && needs_no_redisplay (w))
(gdb) p !just_this_one_p
$6 = 1
(gdb) p needs_no_redisplay (w)
$7 = false
(gdb) p REDISPLAY_SOME_P ()
$8 = 0
(gdb) p !w->redisplay
$9 = 0
(gdb) p !w->update_mode_line
$10 = 0
(gdb) p !f->face_change
$11 = 1
(gdb) p !f->redisplay
$12 = 0
(gdb) p !buffer->text->redisplay
$13 = 0
(gdb) p window_point (w) == w->last_point
$14 = 0
> Next, this:
>
>> 20520 if (current_matrix_up_to_date_p
>> (gdb)
>> 20540 else if (w->start_at_line_beg
>
> indicates that current_matrix_up_to_date_p is false, but why is that?
> It is set around line 20160:
>
> current_matrix_up_to_date_p
> = (w->window_end_valid
> && !current_buffer->clip_changed
> && !current_buffer->prevent_redisplay_optimizations_p
> && !window_outdated (w)
> && !composition_break_at_point
> && !hscrolling_current_line_p (w));
>
> Which one of these causes current_matrix_up_to_date_p to become false?
> Or maybe it starts at true, but becomes false below:
>
> /* When windows_or_buffers_changed is non-zero, we can't rely
> on the window end being valid, so set it to zero there. */
> if (windows_or_buffers_changed)
> {
> /* If window starts on a continuation line, maybe adjust the
> window start in case the window's width changed. */
> if (XMARKER (w->start)->buffer == current_buffer)
> compute_window_start_on_continuation_line (w);
>
> w->window_end_valid = false;
> /* If so, we also can't rely on current matrix
> and should not fool try_cursor_movement below. */
> current_matrix_up_to_date_p = false;
> }
(gdb) break xdisp.c:20520
Breakpoint 5 at 0x5555555f92ea: file /home/steve/src/emacs/emacs-master/src/xdisp.c, line 20520.
(gdb) c
Continuing.
Thread 1 "emacs" hit Breakpoint 3, redisplay_window (window=XIL(0x555555c828e5),
just_this_one_p=just_this_one_p <at> entry=false)
at /home/steve/src/emacs/emacs-master/src/xdisp.c:21195
21195 if (CHARPOS (opoint) < BEGV)
(gdb) c
Continuing.
Thread 1 "emacs" hit Breakpoint 4, redisplay_window (window=XIL(0x555555c826c5),
just_this_one_p=just_this_one_p <at> entry=false)
at /home/steve/src/emacs/emacs-master/src/xdisp.c:20081
20081 if (!just_this_one_p && needs_no_redisplay (w))
(gdb)
Continuing.
Thread 1 "emacs" hit Breakpoint 5, redisplay_window (window=XIL(0x555555c826c5),
just_this_one_p=just_this_one_p <at> entry=false)
at /home/steve/src/emacs/emacs-master/src/xdisp.c:20520
20520 if (current_matrix_up_to_date_p
(gdb) p current_matrix_up_to_date_p
$15 = <optimized out>
(gdb) p w->window_end_valid
$16 = false
(gdb) p !current_buffer->clip_changed
$17 = 1
(gdb) p !current_buffer->prevent_redisplay_optimizations_p
$18 = 0
(gdb) p !window_outdated (w)
$19 = 0
(gdb) p !composition_break_at_point
$20 = 1
(gdb) p !hscrolling_current_line_p (w)
$21 = 1
(gdb) n
20540 else if (w->start_at_line_beg
(gdb) p current_matrix_up_to_date_p
$22 = <optimized out>
> Next,
>
>> (gdb)
>> 20556 else if ((tem = try_window_id (w)) != 0)
>> (gdb)
>
> indicates that try_window_id returned zero. I'd be interested to know
> why.
(gdb) n
20556 else if ((tem = try_window_id (w)) != 0)
(gdb) p try_window_id (w)
$23 = 0
(gdb) break xdisp.c:22148
Breakpoint 6 at 0x5555555f709a: file /home/steve/src/emacs/emacs-master/src/xdisp.c, line 22149.
(gdb) c
Continuing.
Thread 1 "emacs" hit Breakpoint 6, try_window_id (w=w <at> entry=0x555555c826c0)
at /home/steve/src/emacs/emacs-master/src/xdisp.c:22149
22149 {
(gdb) n
22150 struct frame *f = XFRAME (w->frame);
(gdb)
22155 if (is_tty_root_frame_with_visible_child (f))
(gdb)
22158 struct glyph_matrix *current_matrix = w->current_matrix;
(gdb)
22159 struct glyph_matrix *desired_matrix = w->desired_matrix;
(gdb)
22166 ptrdiff_t delta = 0, delta_bytes = 0, stop_pos;
(gdb)
22191 SET_TEXT_POS_FROM_MARKER (start, w->start);
(gdb)
22195 if (MINI_WINDOW_P (w))
(gdb) 22199 if (windows_or_buffers_changed || f->cursor_type_changed)
(gdb)
redisplay_window (window=XIL(0x555555c826c5),
just_this_one_p=just_this_one_p <at> entry=false)
at /home/steve/src/emacs/emacs-master/src/xdisp.c:20570
20570 else if (CHARPOS (startp) >= BEGV
>> 20570 else if (CHARPOS (startp) >= BEGV
>
> This indicates that one of the conditions below
>
> else if (CHARPOS (startp) >= BEGV
> && CHARPOS (startp) <= ZV
> && PT >= CHARPOS (startp)
> && (CHARPOS (startp) < ZV
> /* Avoid starting at end of buffer. */
> || CHARPOS (startp) == BEGV
> || !window_outdated (w)))
>
> yields false, but which one?
(gdb) p CHARPOS (startp) >= BEGV
$24 = 1
(gdb) p CHARPOS (startp) <= ZV
$25 = 1
(gdb) p PT >= CHARPOS (startp)
$26 = 1
(gdb) p CHARPOS (startp) < ZV
$27 = 0
(gdb) p CHARPOS (startp) == BEGV
$28 = 0
(gdb) p !window_outdated (w)
$29 = 0
>> 20684 if ((0 < scroll_conservatively
>> (gdb)
>> 20732 init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
>> (gdb)
>> 20733 it.current_y = it.last_visible_y;
>> (gdb)
>> 20734 if (centering_position < 0)
>
> Here we arrive at a place where redisplay_window decided it must
> redraw the window by recentering point in the viewport, and that's
> what happens afterwards. So the above questions should explain why
> this happens, instead of the expected result: redisplay_window
> deciding that the window's display is already up-to-day and needs no
> display changes.
Does the trace answer your questions? If so, can you connect the dots
for me, because I still don't see why the recentering happens. If not,
can you tell what's still missing?
Steve Berman
GNU bug tracking system
Copyright (C) 1999 Darren O. Benham,
1997,2003 nCipher Corporation Ltd,
1994-97 Ian Jackson.