bug#27427: 26.0.50; Native line numbers lead to display error in company-mode popup

[martin rudalics <rudalics@gmx.at>]

 > Yes, but I presumed it will do it the same way company-mode reacts to
 > such changes today.  Currently. company-mode recomputes and moves its
 > overlay; it will instead recompute the text and its position.

To my knowledge ‘company-mode’ and all packages trying to solve similar
display problems are based on idle timers.  The overlay is moved, if
necessry, when the timeout expires.  I'm not sure whether it makes sense
to use a timer-less approach.  If it does, we should allow moving the
overlay right after recalculating the frame's glyph matrix on TTYs.
Whether removing timers makes sense for GUIs is yet another question.

In this context the ‘window-text-change-functions’ I mentioned earlier
seems completely useless.  Here doing for example

(add-hook 'window-text-change-functions 'ignore)

just gets me into a loop and I have to kill Emacs.  Also, it's not clear
to me why this hook is considered abnormal and how to identify from
Elisp the window that is redisplayed at the time the hook is run.  The
fact that the window's buffer is current at that time is hardly useful.
So I think we should either remove that hook or completely redesign it.

martin