bug#51550: 29.0.50; Customize Group INS buttons sometimes don't have a left box line

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: stefan@marxist.se,  stephen.berman@gmx.net,  51550@debbugs.gnu.org
> Date: Tue, 28 Dec 2021 08:37:28 +0800
> 
> (insert #("foo\nfoo " 0 4 (face widget-field) 4 7 (face custom-button)))
> 
> The second foo will be missing a start box line, which is the bug here.

I didn't yet look seriously enough at that, but: I can only reproduce
this problem on the master branch; Emacs 28 works as expected.  Is
that what you see as well?  If so, then it is strange that you think
the problem happens in the code that wasn't touched at all between
Emacs 28 and Emacs 29, AFAICT.  Most probably, the root cause is
somewhere else.