bug#61667: 29.0.60; Failure to redisplay

[Gregory Heytings]

> > > To avoid this measurement affecting the delay itself, as we saw with 
> > > printfs and trace-redisplay, the timings should be sent via pipe to a 
> > > file, not to the screen.
> >
> > If they indeed don't affect the measurement when they are sent to a 
> > file, it is probably possible to sent them to the screen without 
> > affecting the measurement, by calling 'tail -f' on the file in which 
> > they are recorded in another terminal.
>
> Yes, I suppose this can work, if the new terminal is positioned far away 
> from Emacs's window.
>
> None of the new proposed tests depend on me being able to monitor the 
> output in real time, though.

If you want to measure the latency between the moment an XFlush is issued 
by Emacs and the moment you actually see the buffer contents of the buffer 
on screen, I think you could screencast your repro and use the recorded 
video to make that measurement (unless screencasting eliminates the 
problem, too...).