Re: Why do idle timers trigger redisplay?

[Eli Zaretskii <eliz@gnu.org>]

> Date:  Thu, 11 Feb 2016 16:14:44 -0800
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  22404@debbugs.gnu.org
> 
> Question, please:  Is it a necessary evil that the `timer-idle-list` must always trigger a redisplay?

Emacs does another redisplay cycle each time there was some kind of
input.  The logic of this is that any input could potentially affect
what is displayed (the job of figuring out what, if anything, does
need to be redrawn is integral part of what redisplay does, and it
might as well conclude that no redisplay is actually needed).

Code run by timers is one such kind of input, since any code run by
timers runs with the purpose of doing something with Emacs data:
buffers, strings, windows, frames, etc.

Other kinds of input include: keyboard input, input from subprocesses,
mouse gestures, events that come from sources like D-bus and file
notifications, etc.  They all cause a cycle of redisplay.