bug#38038: 27.0.50; Emacs freezes opening tabs.

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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 removed the actual call in extend_face_to_end_of_line to merge_face
and the issue is still there. And it is actually the only place where
the :extend attribute is used now; the rest of the code does pretty much
the same than before.

Just in case:

I also made a rebase locally and removed all the commits related with
the extend attribute and the issue is still there. So it seems not be
related at all with the extend attribute...

Any help here?. I am out of ideas. (and I don't have a clue about gc).

Ergus