bug#53871: 29.0.50; Emacs freezes with new child-frame option

[Arash Esbati]

Hi all,

I'm trying the new option child-frame available for
`show-paren-context-when-offscreen' (introduced in 6e5d79c048) and I'm
running into an issue where Emacs freezes.  Steps to trigger:

1) emacs -Q
2) eval (setq show-paren-context-when-offscreen t) in scratch
3) M-x find-library RET paren RET
4) M-g c 13902 RET
5) With the mouse cursor, grab the scroll bar and move down so far to
   get the context
   'Matches (defun show-paren--show-context-in-child-frame (text)'
   in the echo area.
6) eval (setq show-paren-context-when-offscreen 'child-frame) in scratch
7) Repeat 5) and Emacs freezes

This is with repo-version 90eb6a7fe4 on Win10.

Best, Arash