bug#27647: 26.0.50; Line numbers implemented natively disappear momentarily when frame out of focus

[Noam Postavsky]

On Wed, Jul 12, 2017 at 10:44 AM, Eli Zaretskii <eliz@gnu.org> wrote:
>> From: Kaushal Modi <kaushal.modi@gmail.com>
>> Date: Tue, 11 Jul 2017 20:08:47 +0000
>> Cc: 27647@debbugs.gnu.org
>>
>> This is what I see (I was able to record the moment when the lines 
>> reappeared on clicking anywhere in the
>> window): http://i.imgur.com/5C5FmGS.gifv
>>
>> When the above happened, the point was in a Fundamental Mode buffer window 
>> and the other window had
>> this file in emacs-lisp-mode. I do not have display-line-numbers set to t 
>> globally; so fundamental-mode has
>> that value at nil and the elisp window had that set to t. Again the line num 
>> disappearance happened when the
>> emacs frame was out of focus.
>>
>> Interestingly the variable watcher did not get triggered.
>
> I guess the variable gets changed by one of the ways that Lisp-level
> watchers cannot catch.

Perhaps 'watch Vdisplay_line_numbers' in gdb would help?