bug#38038: 27.0.50; Emacs freezes opening tabs.

[martin rudalics]

> These hangs occur not only with tab-bar, but also with menu-bar
> when tab-bar is not enabled.  These hangs can be reproduced by
> configuring --with-x-toolkit=no and running emacs -Q.
> Then evaluating this causes hangs:
>
>    (set-face-attribute 'menu nil :weight 'ultra-bold :underline t)

On Windows

(set-face-attribute 'default nil :underline t)

suffices to make it hang in the tool bar redisplay code.  I have the
guts feeling that it's related to the new :extend mechanism because it
works in Emacs 26.

martin