Re: Aborting display. Is this possible?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 19 Oct 2014 14:17:13 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> A lisp program can influence an ongoing display operation by setting
> redisplay-dont-pause to nil.  When input arrives, the displaying is
> immediately paused.

It is not paused, it is aborted.  Emacs cannot "pause" redisplay and
then "resume" it.

> However, at the next opportunity, the displaying carries on where it
> left off, rather than being aborted.

I don't understand what you mean by "carries on where it left off".
Emacs doesn't record where redisplay "left off", and cannot resume an
interrupted redisplay cycle.  It simply starts redisplay anew on the
next opportunity.

> Why?  Better to handle a user auto-repeating on the PageDown key.  On my
> system, auto-repeat is around 35-40 characters per second, on a window
> with 65 lines.  In my favourite mode, display can not keep up.  If I set
> redisplay-dont-pause to nil and hold down PageDown, what the user sees
> is frozen for several seconds whilst display determines the display of
> every intermediate buffer position (which isn't going to get displayed
> anyway) before finally reaching point.

That's not what happens.  What does happen is that redisplay _tries_
several times to do its job, right after each PageDown keypress, each
time starting with the value of point it sees, then abandoning that
attempt because input arrives before it could finish.

Emacs never examines portions of the buffer that don't need to be on
the screen (or a few lines of text above or below that).  It always
starts with the current value of point, and works from that.  So
intermediate buffer positions will never be examined.

> Would it be possible to enhance display such that incoming input would
> abort the current display operation?

It does already.  Emacs simply doesn't know any better.