bug#55779: 29.0.50; child frame

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

drshapeless <drsl@drshapeless.com> writes:

> When I build emacs --with-x-toolkit=gtk3, the child frame generated by
> corfu completion candidates are blocked by a blank overlay. I can
> confirm this is only a gtk3 related issue, because I have built emacs
> --with-x-toolkit=athena or --with-x-toolkit=no without facing the same
> issue. Also, this issue does not occur on older stable branch,
> e.g. 28.1, I can still use gtk3 as the toolkit on older build without
> any issues.
>
> I have reached out to the author of corfu mode. The author says he
> cannot do anything because it is an issue related to emacs child frame
> mechanism. And my experiments on different build configurations showed
> the same conclusion.

What does "are blocked by a blank overlay" mean?  It is technically
impossible for anything (such as an overlay) in the parent frame to
obscure the contents of the child frame, so I'm afraid I don't
understand what you mean by "blocked"?

Secondly, what happens if you start Emacs with
"GDK_CORE_DEVICE_EVENTS=1" in the environment?

Thanks.