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

[Eli Zaretskii]

> Date: Mon, 01 Nov 2021 21:52:17 +0800
> From:  Po Lu via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> 
> Run emacs -Q.  Do M-x customize-group RET files RET, expand "After Save
> Hook", then click "INS" twice.  The second INS button will not have a
> left box line, which is visually inconsistent with the first, and with
> how buttons work in general across all X-Windows applications.

I cannot reproduce this.  Maybe this is specific to GTK?