bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Daniel Clemente]

I have --enable-checking=yes,glyphs enabled, and am using emacsclient
without X support but inside a urxvt terminal in X. Built with no
extra patches. Running in gdb.

I was editing Python code when this error happened. Last things I did were:
M-space to remove extra spaces, and by mistake I hit S-space which
enabled scim (input method chooser, a small widget appearing on top
and stealing window-manager focus), then I disabled it again.
This information may be bogus since these operations work fine when I
retest them.
I wasn't doing anything weird like stress-testing it, resizing
windows, changing fonts/resolution, …

I don't know how to reproduce this yet. Feel free to close if it
doesn't clearly show a possible bug.
I didn't have time to research the crash more in detail; and I forgot
"bt full". I'll do it if I can reproduce it.

Note that the window is called "Folder". I'm certain it refers to
Wanderlust's (Emacs mail reader) main view which shows the mailboxes
(inbox, drafts, etc.), which I had open at that time in a different
frame (not visible, i.e. it was in a different window manager
workspace and I wasn't working with it). I don't know why it relates
to the crash; maybe I switched to that frame for a bit without
noticing?
Proof that Wanderlust's window is called Folder: (get-buffer-window)
returns something like: #<window 19 on Folder>



[Detaching after vfork from child process 23040]
[Detaching after vfork from child process 23097]
[Detaching after vfork from child process 24446]
[Detaching after vfork from child process 24447]
[Detaching after vfork from child process 24454]
[Detaching after vfork from child process 24455]

xdisp.c:21783: Emacs fatal error: assertion failed: w->window_end_valid

Breakpoint 1, terminate_due_to_signal (sig=6,
backtrace_limit=2147483647) at emacs.c:443
443      signal (sig, SIG_DFL);


(gdb) bt
#0  terminate_due_to_signal (sig=6, backtrace_limit=2147483647) at emacs.c:443
#1  0x00005555557350bb in die (msg=0x5555558527c0 "w->window_end_valid",
    file=0x555555850b08 "xdisp.c", line=21783) at alloc.c:8082
#2  0x00005555555dcd5f in find_first_unchanged_at_end_row (w=0x5555622c4f68,
    delta=0x7fffffff7b18, delta_bytes=0x7fffffff7b10) at xdisp.c:21783
#3  0x00005555555de527 in try_window_id (w=0x5555622c4f68) at xdisp.c:22347
#4  0x00005555555d93ad in redisplay_window
(window=XIL(0x5555622c4f6d), just_this_one_p=false)
    at xdisp.c:20438
#5  0x00005555555d0991 in redisplay_window_0
(window=XIL(0x5555622c4f6d)) at xdisp.c:18016
#6  0x000055555576c7b0 in internal_condition_case_1
(bfun=0x5555555d094f <redisplay_window_0>,
    arg=XIL(0x5555622c4f6d), handlers=XIL(0x7ffff1f56cdb),
    hfun=0x5555555d082d <redisplay_window_error>) at eval.c:1637
#7  0x00005555555d0803 in redisplay_windows
(window=XIL(0x5555622c4f6d)) at xdisp.c:17985
#8  0x00005555555cf382 in redisplay_internal () at xdisp.c:17384
#9  0x00005555555cfed6 in redisplay_preserve_echo_area (from_where=9)
at xdisp.c:17747
#10 0x00005555557ed8a7 in wait_reading_process_output (time_limit=30,
nsecs=0, read_kbd=-1,
    do_display=true, wait_for_cell=XIL(0), wait_proc=0x0,
just_wait_proc=0) at process.c:5434
#11 0x0000555555594eb9 in sit_for (timeout=make_fixnum(30),
reading=true, display_option=1)
    at dispnew.c:6318
#12 0x0000555555695250 in read_char (commandflag=1,
map=XIL(0x555564f84233), prev_event=XIL(0),
    used_mouse_menu=0x7fffffffd6df, end_time=0x0) at keyboard.c:2923
#13 0x00005555556a67ff in read_key_sequence (keybuf=0x7fffffffd8f0,
prompt=XIL(0),
    dont_downcase_last=false, can_return_switch_frame=true,
fix_current_buffer=true,
    prevent_redisplay=false, disable_text_conversion_p=false) at
keyboard.c:10728
#14 0x0000555555690ee7 in command_loop_1 () at keyboard.c:1429
#15 0x000055555576c6cf in internal_condition_case (bfun=0x555555690ab8
<command_loop_1>,
    handlers=XIL(0x90), hfun=0x55555568ff63 <cmd_error>) at eval.c:1613
#16 0x0000555555690696 in command_loop_2 (handlers=XIL(0x90)) at keyboard.c:1168
#17 0x000055555576bafc in internal_catch (tag=XIL(0xfc60),
func=0x55555569066c <command_loop_2>,
    arg=XIL(0x90)) at eval.c:1292
#18 0x0000555555690628 in command_loop () at keyboard.c:1146
#19 0x000055555568fa05 in recursive_edit_1 () at keyboard.c:754
#20 0x000055555568fc31 in Frecursive_edit () at keyboard.c:837
#21 0x000055555568b698 in main (argc=2, argv=0x7fffffffdf18) at emacs.c:2621

Lisp Backtrace:
"redisplay_internal (C function)" (0x0)
(gdb)

(gdb) frame 4
#4  0x00005555555d93ad in redisplay_window
(window=XIL(0x5555622c4f6d), just_this_one_p=false)
    at xdisp.c:20438
20438      else if ((tem = try_window_id (w)) != 0)


(gdb) pp window
#<window 85 on Folder>


(other random debugging done here)

(gdb) cont
Continuing.
Fatal error 6: Aborted
Backtrace:
/opt/dc/emacs/bin/emacs(+0x1691ad)[0x5555556bd1ad]
/opt/dc/emacs/bin/emacs(+0x1344a8)[0x5555556884a8]
/opt/dc/emacs/bin/emacs(+0x1e10bb)[0x5555557350bb]
/opt/dc/emacs/bin/emacs(+0x88d5f)[0x5555555dcd5f]
/opt/dc/emacs/bin/emacs(+0x8a527)[0x5555555de527]
/opt/dc/emacs/bin/emacs(+0x853ad)[0x5555555d93ad]
/opt/dc/emacs/bin/emacs(+0x7c991)[0x5555555d0991]
/opt/dc/emacs/bin/emacs(+0x2187b0)[0x55555576c7b0]
/opt/dc/emacs/bin/emacs(+0x7c803)[0x5555555d0803]
/opt/dc/emacs/bin/emacs(+0x7b382)[0x5555555cf382]
/opt/dc/emacs/bin/emacs(+0x7bed6)[0x5555555cfed6]
/opt/dc/emacs/bin/emacs(+0x2998a7)[0x5555557ed8a7]
/opt/dc/emacs/bin/emacs(+0x40eb9)[0x555555594eb9]
/opt/dc/emacs/bin/emacs(+0x141250)[0x555555695250]
/opt/dc/emacs/bin/emacs(+0x1527ff)[0x5555556a67ff]
/opt/dc/emacs/bin/emacs(+0x13cee7)[0x555555690ee7]
/opt/dc/emacs/bin/emacs(+0x2186cf)[0x55555576c6cf]
/opt/dc/emacs/bin/emacs(+0x13c696)[0x555555690696]
/opt/dc/emacs/bin/emacs(+0x217afc)[0x55555576bafc]
/opt/dc/emacs/bin/emacs(+0x13c628)[0x555555690628]
/opt/dc/emacs/bin/emacs(+0x13ba05)[0x55555568fa05]
/opt/dc/emacs/bin/emacs(+0x13bc31)[0x55555568fc31]
/opt/dc/emacs/bin/emacs(+0x137698)[0x55555568b698]
/lib/x86_64-linux-gnu/libc.so.6(+0x2724a)[0x7ffff544624a]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7ffff5446305]
/opt/dc/emacs/bin/emacs(+0x30831)[0x555555584831]

Program received signal SIGABRT, Aborted.
__pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6,
    no_tid=no_tid@entry=0) at ./nptl/pthread_kill.c:44
44    ./nptl/pthread_kill.c: No such file or directory.



In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu) of 2024-05-29 built on sonn
Repository revision: d9512da49514623ef3e35524dc894c06f2c0ce20
Repository branch: master
System Description: Devuan GNU/Linux 5 (daedalus)

Configured using:
 'configure --prefix=/opt/dc/emacs/ --without-dbus --with-tiff=no --without-tiff
 --without-libsystemd --without-dbus --with-mailutils
--with-native-compilation --with-x-toolkit=no
 --without-imagemagick --without-xft --without-harfbuzz
--without-freetype --without-libotf
 --without-xwidgets --without-xpm --without-jpeg --without-gif
--without-png --without-webp
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--enable-checking=yes,glyphs 'CFLAGS=-g3 -O0''

Configured features:
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PDUMPER SECCOMP SOUND SQLITE3
THREADS XIM ZLIB

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bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Eli Zaretskii]

> From: Daniel Clemente <n142857@gmail.com>
> Date: Thu, 30 May 2024 09:51:20 +0000
> 
> I have --enable-checking=yes,glyphs enabled, and am using emacsclient
> without X support but inside a urxvt terminal in X. Built with no
> extra patches. Running in gdb.
> 
> I was editing Python code when this error happened. Last things I did were:
> M-space to remove extra spaces, and by mistake I hit S-space which
> enabled scim (input method chooser, a small widget appearing on top
> and stealing window-manager focus), then I disabled it again.
> This information may be bogus since these operations work fine when I
> retest them.
> I wasn't doing anything weird like stress-testing it, resizing
> windows, changing fonts/resolution, …
> 
> I don't know how to reproduce this yet. Feel free to close if it
> doesn't clearly show a possible bug.

Not only does it not show a possible bug, it actually shows an
impossible condition.  I feel like I must be missing something here,
because this simply cannot happen, if my reading of the code is
correct.  See below.

> (gdb) bt
> #0  terminate_due_to_signal (sig=6, backtrace_limit=2147483647) at emacs.c:443
> #1  0x00005555557350bb in die (msg=0x5555558527c0 "w->window_end_valid",
>     file=0x555555850b08 "xdisp.c", line=21783) at alloc.c:8082
> #2  0x00005555555dcd5f in find_first_unchanged_at_end_row (w=0x5555622c4f68,
>     delta=0x7fffffff7b18, delta_bytes=0x7fffffff7b10) at xdisp.c:21783

This points to the following assertion:

  static struct glyph_row *
  find_first_unchanged_at_end_row (struct window *w,
				   ptrdiff_t *delta, ptrdiff_t *delta_bytes)
  {
    struct glyph_row *row;
    struct glyph_row *row_found = NULL;

    *delta = *delta_bytes = 0;

    /* Display must not have been paused, otherwise the current matrix
       is not up to date.  */
    eassert (w->window_end_valid); <<<<<<<<<<<<<<<<<<<<<<

However, find_first_unchanged_at_end_row is called from try_window_id:

> #3  0x00005555555de527 in try_window_id (w=0x5555622c4f68) at xdisp.c:22347

And that function already checked that w->window_end_valid is
non-zero, several dozens of lines before that:

    /* Verify that display wasn't paused.  */
    if (!w->window_end_valid)
      GIVE_UP (8);

GIVE_UP returns from the function.  So how come w->window_end_valid
became zero between that test and the assertion, Emacs being a
single-threaded program??  What am I missing here?  The only possible
explanation I could think of is some compiler bug that causes it to
emit incorrect code, or maybe some hardware problem with memory.  Both
possibilities are almost unthinkable.

Any ideas, even crazy ones, are welcome.

bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Daniel Clemente]

> However, find_first_unchanged_at_end_row is called from try_window_id:
> > #3  0x00005555555de527 in try_window_id (w=0x5555622c4f68) at xdisp.c:22347
> And that function already checked that w->window_end_valid is
> non-zero, several dozens of lines before that:
>     /* Verify that display wasn't paused.  */
>     if (!w->window_end_valid)
>       GIVE_UP (8);

Between this check (the one "several dozens of lines before that"),
and the call to find_first_unchanged_at_end_row, something has the
side effect of changing window_end_valid to false. In particular:


  if (last_unchanged_at_beg_row)
    {
      /* Avoid starting to display in the middle of a character, a TAB
     for instance.  This is easier than to set up the iterator
     exactly, and it's not a frequent case, so the additional
     effort wouldn't really pay off.  */
      while ((MATRIX_ROW_ENDS_IN_MIDDLE_OF_CHAR_P (last_unchanged_at_beg_row)
          || last_unchanged_at_beg_row->ends_in_newline_from_string_p)
         && last_unchanged_at_beg_row > w->current_matrix->rows)
    --last_unchanged_at_beg_row;

      if (MATRIX_ROW_ENDS_IN_MIDDLE_OF_CHAR_P (last_unchanged_at_beg_row))
    GIVE_UP (17);

      if (! init_to_row_end (&it, w, last_unchanged_at_beg_row))
    GIVE_UP (18);


Before that last init_to_row_end, w->window_end_valid was true, and,
after it, it was false.
I saw it through fprintfs added before and after the line.
It's not always like that; sometimes it just stays true all through. I
still don't know the conditions to reproduce this, but I have seen
this bug 4 or 5 times yesterday+today.

So it must be a side effect of init_to_row_end.

I didn't have to read its code yet.


By the way, could this comment in try_window_id explain what is
happening? Maybe this part needs to be moved earlier.

  /* The redisplay iterations in display_line above could have
     triggered font-lock, which could have done something that
     invalidates IT->w window's end-point information, on which we
     rely below.  E.g., one package, which will remain unnamed, used
     to install a font-lock-fontify-region-function that called
     bury-buffer, whose side effect is to switch the buffer displayed
     by IT->w, and that predictably resets IT->w's window_end_valid
     flag, which we already tested at the entry to this function.
     Amply punish such packages/modes by giving up on this
     optimization in those cases.  */
  if (!w->window_end_valid)
    {
      clear_glyph_matrix (w->desired_matrix);
      return -1;
    }

bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Eli Zaretskii]

> From: Daniel Clemente <n142857@gmail.com>
> Date: Fri, 31 May 2024 10:08:18 +0000
> Cc: 71274@debbugs.gnu.org
> 
> > However, find_first_unchanged_at_end_row is called from try_window_id:
> > > #3  0x00005555555de527 in try_window_id (w=0x5555622c4f68) at xdisp.c:22347
> > And that function already checked that w->window_end_valid is
> > non-zero, several dozens of lines before that:
> >     /* Verify that display wasn't paused.  */
> >     if (!w->window_end_valid)
> >       GIVE_UP (8);
> 
> Between this check (the one "several dozens of lines before that"),
> and the call to find_first_unchanged_at_end_row, something has the
> side effect of changing window_end_valid to false. In particular:
> 
> 
>   if (last_unchanged_at_beg_row)
>     {
>       /* Avoid starting to display in the middle of a character, a TAB
>      for instance.  This is easier than to set up the iterator
>      exactly, and it's not a frequent case, so the additional
>      effort wouldn't really pay off.  */
>       while ((MATRIX_ROW_ENDS_IN_MIDDLE_OF_CHAR_P (last_unchanged_at_beg_row)
>           || last_unchanged_at_beg_row->ends_in_newline_from_string_p)
>          && last_unchanged_at_beg_row > w->current_matrix->rows)
>     --last_unchanged_at_beg_row;
> 
>       if (MATRIX_ROW_ENDS_IN_MIDDLE_OF_CHAR_P (last_unchanged_at_beg_row))
>     GIVE_UP (17);
> 
>       if (! init_to_row_end (&it, w, last_unchanged_at_beg_row))
>     GIVE_UP (18);
> 
> 
> Before that last init_to_row_end, w->window_end_valid was true, and,
> after it, it was false.

Thanks, but please find where it changes inside the call to
init_to_row_end, because I couldn't see anything obvious in the code
involved in that call.  There's some factor at work here that we need
to identify and understand.

(It is easy to add some band-aid without such an understanding, but
I'm not yet prepared to do that.)

> I saw it through fprintfs added before and after the line.
> It's not always like that; sometimes it just stays true all through. I
> still don't know the conditions to reproduce this, but I have seen
> this bug 4 or 5 times yesterday+today.
> 
> So it must be a side effect of init_to_row_end.

Yes, but where exactly in that function or the ones it calls?

> By the way, could this comment in try_window_id explain what is
> happening? Maybe this part needs to be moved earlier.

Maybe.  But I'd like to see a backtrace with some of those guilty
parties in the callstack, before I do that.

bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Daniel Clemente]

> Thanks, but please find where it changes inside the call to
> init_to_row_end, because I couldn't see anything obvious in the code
> involved in that call.  There's some factor at work here that we need
> to identify and understand.

It happens (window_end_valid becoming false) inside init_iterator,  in
init_from_display_pos (called init_to_row_end). Here:

  /* Keep in mind: the call to reseat in init_iterator skips invisible
     text, so we might end up at a position different from POS.  This
     is only a problem when POS is a row start after a newline and an
     overlay starts there with an after-string, and the overlay has an
     invisible property.  Since we don't skip invisible text in
     display_line and elsewhere immediately after consuming the
     newline before the row start, such a POS will not be in a string,
     but the call to init_iterator below will move us to the
     after-string.  */
 init_iterator (it, w, charpos, bytepos, NULL, DEFAULT_FACE_ID);


Inside init_iterator, window_end_valid becomes false during this code
(i.e. it was true just before, and false just after):

  /* If face attributes have been changed since the last redisplay,
     free realized faces now because they depend on face definitions
     that might have changed.  Don't free faces while there might be
     desired matrices pending which reference these faces.  */
  if (!inhibit_free_realized_faces)
    {
      if (face_change)
    {
      face_change = false;
      XFRAME (w->frame)->face_change = 0;
      free_all_realized_faces (Qnil);
    }
      else if (XFRAME (w->frame)->face_change)
    {
      XFRAME (w->frame)->face_change = 0;
      free_all_realized_faces (w->frame);
    }
    }

I don't know yet which branch runs or what happens inside
free_all_realized_faces, since I didn't see this crash recently.

Does this give any hint about how window_end_valid could become false?

In next tests I'll run with the latest git version, including your
recent commit eb9afd558ec (which I know is for something different
—SIGWINCH— but who knows, maybe it was related to this bug).

bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Eli Zaretskii]

> From: Daniel Clemente <n142857@gmail.com>
> Date: Mon, 3 Jun 2024 17:50:47 +0000
> Cc: 71274@debbugs.gnu.org
> 
> > Thanks, but please find where it changes inside the call to
> > init_to_row_end, because I couldn't see anything obvious in the code
> > involved in that call.  There's some factor at work here that we need
> > to identify and understand.
> 
> It happens (window_end_valid becoming false) inside init_iterator,  in
> init_from_display_pos (called init_to_row_end). Here:
> 
>   /* Keep in mind: the call to reseat in init_iterator skips invisible
>      text, so we might end up at a position different from POS.  This
>      is only a problem when POS is a row start after a newline and an
>      overlay starts there with an after-string, and the overlay has an
>      invisible property.  Since we don't skip invisible text in
>      display_line and elsewhere immediately after consuming the
>      newline before the row start, such a POS will not be in a string,
>      but the call to init_iterator below will move us to the
>      after-string.  */
>  init_iterator (it, w, charpos, bytepos, NULL, DEFAULT_FACE_ID);
> 
> 
> Inside init_iterator, window_end_valid becomes false during this code
> (i.e. it was true just before, and false just after):
> 
>   /* If face attributes have been changed since the last redisplay,
>      free realized faces now because they depend on face definitions
>      that might have changed.  Don't free faces while there might be
>      desired matrices pending which reference these faces.  */
>   if (!inhibit_free_realized_faces)
>     {
>       if (face_change)
>     {
>       face_change = false;
>       XFRAME (w->frame)->face_change = 0;
>       free_all_realized_faces (Qnil);
>     }
>       else if (XFRAME (w->frame)->face_change)
>     {
>       XFRAME (w->frame)->face_change = 0;
>       free_all_realized_faces (w->frame);
>     }
>     }
> 
> I don't know yet which branch runs or what happens inside
> free_all_realized_faces, since I didn't see this crash recently.
> 
> Does this give any hint about how window_end_valid could become false?

Yes, it does, thanks.  I installed a fix which hopefully plugs this.

> In next tests I'll run with the latest git version, including your
> recent commit eb9afd558ec (which I know is for something different
> —SIGWINCH— but who knows, maybe it was related to this bug).

I don't think so.

bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Daniel Clemente]

Thanks, well, I'm not seeing these errors anymore. I don't have a
formula to reproduce the bug, but I trust the patch will help and this
bug can be closed.

I don't much knowledge about try_window_id. Could this change create
new situations in which a window isn't fully redisplayed but it
should?
I saw a redisplay issue after resizing the window (mangled text) but I
don't think it's related to this bug because I placed a breakpoint on
your new code and it didn't run.  I posted some details at 71289.









On Mon, 3 Jun 2024 at 18:15, Eli Zaretskii <eliz@gnu.org> wrote:
>
> > From: Daniel Clemente <n142857@gmail.com>
> > Date: Mon, 3 Jun 2024 17:50:47 +0000
> > Cc: 71274@debbugs.gnu.org
> >
> > > Thanks, but please find where it changes inside the call to
> > > init_to_row_end, because I couldn't see anything obvious in the code
> > > involved in that call.  There's some factor at work here that we need
> > > to identify and understand.
> >
> > It happens (window_end_valid becoming false) inside init_iterator,  in
> > init_from_display_pos (called init_to_row_end). Here:
> >
> >   /* Keep in mind: the call to reseat in init_iterator skips invisible
> >      text, so we might end up at a position different from POS.  This
> >      is only a problem when POS is a row start after a newline and an
> >      overlay starts there with an after-string, and the overlay has an
> >      invisible property.  Since we don't skip invisible text in
> >      display_line and elsewhere immediately after consuming the
> >      newline before the row start, such a POS will not be in a string,
> >      but the call to init_iterator below will move us to the
> >      after-string.  */
> >  init_iterator (it, w, charpos, bytepos, NULL, DEFAULT_FACE_ID);
> >
> >
> > Inside init_iterator, window_end_valid becomes false during this code
> > (i.e. it was true just before, and false just after):
> >
> >   /* If face attributes have been changed since the last redisplay,
> >      free realized faces now because they depend on face definitions
> >      that might have changed.  Don't free faces while there might be
> >      desired matrices pending which reference these faces.  */
> >   if (!inhibit_free_realized_faces)
> >     {
> >       if (face_change)
> >     {
> >       face_change = false;
> >       XFRAME (w->frame)->face_change = 0;
> >       free_all_realized_faces (Qnil);
> >     }
> >       else if (XFRAME (w->frame)->face_change)
> >     {
> >       XFRAME (w->frame)->face_change = 0;
> >       free_all_realized_faces (w->frame);
> >     }
> >     }
> >
> > I don't know yet which branch runs or what happens inside
> > free_all_realized_faces, since I didn't see this crash recently.
> >
> > Does this give any hint about how window_end_valid could become false?
>
> Yes, it does, thanks.  I installed a fix which hopefully plugs this.
>
> > In next tests I'll run with the latest git version, including your
> > recent commit eb9afd558ec (which I know is for something different
> > —SIGWINCH— but who knows, maybe it was related to this bug).
>
> I don't think so.

bug#71274: 30.0.50; assertion failed: w->window_end_valid, in find_first_unchanged_at_end_row

[Eli Zaretskii]

> From: Daniel Clemente <n142857@gmail.com>
> Date: Thu, 6 Jun 2024 12:39:39 +0000
> Cc: 71274@debbugs.gnu.org
> 
> Thanks, well, I'm not seeing these errors anymore. I don't have a
> formula to reproduce the bug, but I trust the patch will help and this
> bug can be closed.

Done.

> I don't much knowledge about try_window_id. Could this change create
> new situations in which a window isn't fully redisplayed but it
> should?

No.  try_window_id is an optimization: it tries to avoid some work by
reusing portions of the previously-displayed glyph matrix.  When
try_window_id calls GIVE_UP, it means the optimization cannot be used,
but the result should still be correct, just perhaps slower.