bug#63187: 30.0.50; Tail of longer lines painted after end of nearby lines on macOS

[Kai Ma]

> On Jun 8, 2023, at 20:51, Alan Third <alan@idiocy.org> wrote:
> 
> I've got two things it could be worth trying to see if they make any
> difference.
> Change the CACHE_MAX_SIZE to something greater than 2, probably 4 is a
> good number. That would rule out some sort of locking problem, as it
> would reduce the chance a surface would be re-used before it's been
> sent to the screen.
> 
> The downside of this is probably going to be an increase in "lag"
> because it's possible to have more surfaces "in-flight".
> 
> The other option is to remove this call (nsterm.m:10636):
> 
>   /* Schedule a run of getContext so that if Emacs is idle it will
>      perform the buffer copy, etc.  */
>   [self performSelectorOnMainThread:@selector (getContext)
>                          withObject:nil
>                       waitUntilDone:NO];
> 
> I think this is harmless, but it's in there as I *assume*, with
> absolutely no proof, that it will improve performance and it seems
> possible (but I think unlikely) that it may affect the sequencing of
> surface operations.

Tested locally.  I can confirm that removing performSelectorOnMainThread (with 
CACHE_MAX_SIZE = 2) fixes the problem for me.  I now observe zero glitches or 
tearings!