bug#75056: 31.0.50; tty-child-frames with server / multiple clients possible hangs

[Eli Zaretskii <eliz@gnu.org>]

> From: Len Trigg <lenbok@gmail.com>
> Date: Tue, 24 Dec 2024 18:43:29 +1300
> 
> tty-child-frames does not seem to play well with multiple clients. When we run server-start and allow emacs to
> have multiple clients (e.g. an initial "emacs -nw" and an
> "emacsclient -nw"), using a function that utilizes tty-child-frame (such as M-x when vertico-posframe is
> loaded) in one frame leads the other client to be locked up.
> 
> Steps to reproduce:
> 
> mkdir ~/emacs-test
> Copy the attached init.el into ~/emacs-test/
> emacs -nw --init-directory=~/emacs-test  (the first time will result in packages being installed by elpaca)
> (in another terminal) emacsclient -nw
> Do something to invoke the child frame pop up (e.g. C-x b and select a buffer)
> Switch back to the original emacs
> Do something to invoke the child frame pop up (e.g. C-x b and select a buffer)
> Swap to the other terminal, and note that at some point one client will stop responding
> to user input. (It may take a couple of tries, perhaps with other regular commands interspersed).
> When one client is locked, swap back to the other terminal and exit the client - the original
> client will now accept user input.
> 
> When a client is locked it *does* accept some input (e.g. C-x C-c will exit the client)
> 
> It's possible this is vertico-posframe related, as I can't trigger similar behaviour using transient-posframe.

Gerd, could you perhaps look into this?