bug#81567: 31.0.90; overlay freeze

Hello Eli,

Just to let you know, I have been using the patch for a few days now and
haven't seen any regressions.

On Fri, 7 Aug 2026 at 12:51, Romain Ouabdelkader <
[email protected]> wrote:

> Hello Eli,
>
> > Quite crazy, IMNSHO.
> I forgot to include that in my first email, disclaimer: I used LLM to
> quickly generate test cases to find a simple reproducer of the issue.
>
> I tried the patch, and I confirm that it fixed the issue.
> I don't see any regression right now, I will use it for a few days to see
> if I notice anything.
>
> Thank you!
>
> On Thu, 6 Aug 2026 at 18:29, Eli Zaretskii <[email protected]> wrote:
>
>> > From: Romain Ouabdelkader <[email protected]>
>> > Date: Thu, 6 Aug 2026 17:51:24 +0200
>> >
>> > The minimal reproducer I could get:
>> > - two line overlays with empty 'display
>> > - an overlay with 'before-string & 'display with multiple lines
>>
>> Quite crazy, IMNSHO.
>>
>> > When you scroll up to the beginning of the buffer, Emacs hangs. C-g does
>> > nothing, neither does kill -SIGUSR2.
>> >
>> > Attached is a minimal reproducer:
>> >
>> > 1. emacs -Q -l overlay-scroll-freeze.el -f overlay-scroll-freeze
>> > 2. hold C-p to move point to the start of the buffer
>> > 3. Emacs hangs when reaching the beginning of the buffer
>>
>> Thanks.
>>
>> (Actually, you must make sure you type C-p twice in quick succession
>> at BOB, otherwise there's no hang.)
>>
>> Please try the patch below and see if it solves the problem without
>> introducing regressions elsewhere.
>>
>> > I am pretty sure that the issue started on emacs31; although I didn't
>> > reinstall emacs30 to verify the reproducer.
>>
>> Emacs 30.2 indeed doesn't hang, but it produces garbled display in
>> this case.
>>
>> diff --git a/src/xdisp.c b/src/xdisp.c
>> index 5377d5e..b3227ce 100644
>> --- a/src/xdisp.c
>> +++ b/src/xdisp.c
>> @@ -11298,10 +11298,18 @@ move_it_vertically_backward (struct it *it, int
>> dy)
>>       y-distance.  */
>>    SAVE_IT (it2, *it, it2data);
>>    it2.max_ascent = it2.max_descent = 0;
>> +  ptrdiff_t to_pos = start_pos;
>>    do
>>      {
>> +      /* Avoid inflooping of there's a large display string with several
>> +         embedded newlines, which makes move_it_to stop after START_POS
>> +         but still inside a display or overlay string.  */
>> +      if (IT_CHARPOS (it2) >= start_pos)
>> +       to_pos = -1;
>>        move_it_to (&it2, start_pos, -1, -1, it2.vpos + 1,
>> -                 MOVE_TO_POS | MOVE_TO_VPOS);
>> +                 (to_pos > 0
>> +                   ? (MOVE_TO_POS | MOVE_TO_VPOS)
>> +                  : MOVE_TO_VPOS));
>>      }
>>    while (!(IT_POS_VALID_AFTER_MOVE_P (&it2)
>>            /* If we are in a display string which starts at START_POS,
>>
>