bug#38038: 27.0.50; Emacs freezes opening tabs.

[Ergus]

On Fri, Nov 08, 2019 at 10:21:14AM +0100, martin rudalics wrote:
> > 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

Hi Martin:

I will try a simple check to test if it is relates commenting the call
to merge in extend_face_to_end_of_line. But I don't see it related at
all; the loop seems to be in some gc functions. Remember that the gc has
been changes too since emacs 26.