It's not really predictable. I didn't manage to reproduce the crash under gdb so far, although without gdb it sometimes crashes and sometimes hands under similar conditions. I'll keep running Emacs under gdb and if a crash occurs, will post it here.
> From: Vladimir Nikishkin <lockywolf@gmail.com>
> Date: Sun, 12 Jan 2020 14:49:12 +0800
> Cc: 39081@debbugs.gnu.org
>
> Sorry, my debugging skills are too poor.
>
> So I attached gdb to Emacs, cont'd, then killed scim, and then
> interrupted Emacs by issuing C-c in the gdb prompt,
> and then collected a stack trace. I am not sure I am doing something
> meaningful, as this is the first time I use gdb:
> Here's the stack trace:
Previously, you said that this unoptimized build "crashed, printing
This is GTK bug". But I don't seed any crash here, just interrupted
Emacs because you typed C-c. Does this mean you cannot reproduce the
crash under GDB?