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

[Eli Zaretskii]

> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 27647@debbugs.gnu.org, jonaswestlund101@gmail.com,
>       npostavs@users.sourceforge.net
> 
> It's most probably something related to some hook or timer that you
> set up in your customizations.

Or maybe some feature calls kill-all-local-variables, but doesn't
restore display-line-numbers until later.