On Monday, 27 August 2018 18:35:29 BST Hervé Guillemet wrote:
> Thanks Mick for the suggestions.

> > This sounds like a Xorg drivers problem, but if Xorg has partially
> > crashed due to RAM or power problems, then this cursor problem will
> > go away after you address the hardware issue.
> > 
> >> Scrolling with scrollbars or arrow keys works. Scrolling with
> >> wheels in others apps like terminals works too. No useful message
> >> in syslog, dmesg or console.
> > 
> > What does /var/log/Xorg.0.log reveal?
> 
> Nothing either.
> 
> In the meantime I found another application triggering the problem (some
> image display software), so this is not specific to Gecko.

There are various tests, from glxgears to glmark2 to Unigine Benchmark, which 
will put a graphics card through its paces and of course test the video driver 
with it.

If by running any of these you get the same symptoms I would think the problem 
is specific to your GPU/driver and perhaps how it uses shared memory - hence 
the beep. You can also see if there is a momentary jump in memory usage by 
using vmstat, watch -d ipcs -m, watch -d cat /proc/meminfo, et al.

Glitches like these usually go away after a while, when the application, Xorg 
drivers and occasionally the kernel get updated to a later version.  Other 
than that you could start debugging apps and drivers, which often ends up 
being a ping-pong between video driver devs, app devs and desktop devs.

I'm not I can offer more practical help, but others more knowledgeable in this 
field may be able to contribute.
-- 
Regards,
Mick