Re: jumpy scrolling on the Mac

On Mon, 20 May 2024 18:48:20 -0400
"John Selverian" <[email protected]> wrote:

> I'm running XQuartz 2.8.5 (xorg-server 21.1.6) and the scrolling
> is very jumpy. The text gets blurred until the scrolling stops.
> Other apps not using X11 are fine when they scroll.
> 
>  
> 
> I posted this to the XQuartz site with a short video:
> 
>  
> 
>  
> 
> https://github.com/XQuartz/XQuartz/issues/397#issuecomment-212073
> 4621

I can't say what the problem is, but my gut feeling says it has something
to do with events piling up somewhere.

FOX tries to be very efficient processing events: we're pulling the event
queue dry, and only start drawing when we catch up.  XExpose events are
piled into the "repaint list"; rectangles which touch or overlap are
consolidated, if the new area is less than the sum of the two old
areas.

So, onces it starts painting, it actually paints less rectangles than
originally were received.

If a widget gets scrolled, parts of the window may have not getting
repainted yet.  FOX solves this by elongating the to-be-repainted
areas in the direction of the scrolling, and bitblits the rest.

Thus, we keep compounding dirty-rectangles until paint-time.

This works pretty well, and prevents processing of the event loop
from falling behind.

This may feel counter-intuitive, but redrawing is often more expensive
than processing regular events, so deferring this until we've caught up
with the event stream means we're never in the position of redrawing
something that we may ALREADY know to be have to be redrawn AGAIN. 
[think of multiple consecutive scrolls].

So what are we seeing in the video? we're seeing LOTS off scrolling,
and the system keeps piling up (or growing) repaint rectangles, not
being able to catch up.

Possible causes:

	1) Extremely high event rate from mouse?

	2) Slow drawing?

	3) Very high refresh-rate screens?

	4) Other callbacks going on preventing the system to reach
	   the "idle" state, i.e. select() or pselect() returning
	   with NO events. 

On my i5-8500T with 165 Hz screen (2K), using iGPU, it has no trouble
whatsoever to smoothly keep up.  

So it must have something to do with how Apple does things, or may 
depend on something going on inside the application.

To rule out your application, may load a bunch of text into the Adie
text editor and scroll it vigourously. If you seen the same problem,
it may be the Apple issue; if the problem is confined to your app,
then some profiling may help.


   -- JVZ