I tried and the bug can happen with breakpoint set. Haven't got a backtrace though.
> From: Amai Kinono <amaikinono@gmail.com>
> Date: Sun, 15 Aug 2021 20:12:26 +0800
> Cc: 44448@debbugs.gnu.org
>
> I'm on the latest commit (55772baee1627571c0814cf2d666fb 3b963ff591) now.
> Unfortunately the bug can still be reproduced on my side.
If you can still reproduce it, show me the backtrace from
signal_or_quit when the buffer is scrolled from EOB to BOB. That
backtrace should tell us which code still needs to be fixed.
That is, set a breakpoint in signal_or_quit with the following
commands:
backtrace
print current_buffer->pt
continue
end
then run your recipe and press C-g until you see the problem happen.
Then post the last few backtraces here.
I can no longer reproduce the problem on my system, so I cannot do
this myself.