Latest Firefox, Linux, NVIDIA drivers, WebRender enabled => 100% CPU usage when monitor is off (DPMS)

Current setup:

 

• Thinkpad P72 with NVIDIA Quadro P600
• Distro: openSUSE Tumbleweed
• Kernel 5.6.2 (also happened with older kernels)
• NVIDIA drivers 440.82 (also happened with older versions)
• Xorg 1.20.8
• Firefox 74.0.1 (from standard distro repo. Also happened with older versions)
• WebRender enabled (gfx.webrender.all=true)
• Desktop: Plasma 5.18.4 with compositor, or i3+picom (it does not matter if compositor is enabled or not)
• using an external 4K monitor over Thunderbolt 3 (DisplayPort)

 

The problem:

 

Whenever my monitor go to sleep via DPMS (either automatically or via hotkey triggering xset dpms force off), after a few seconds, Firefox systematically takes 100% CPU usage until I wake up the monitor where CPU usage goes back to normal. I can hear the CPU usage (laptop fans suddenly kick in) and see it via htop in a remote ssh console.

 

This issue does not happen if either:

 

• I disable WebRender. WebRender otherwise works fine and makes scrolling much smoother than without it.
• I do not use the NVIDIA driver and use my Intel UHD 630 instead with the modesetting driver (with WebRender enabled)

 

=> this issue is specific to NVIDIA and Firefox when using OpenGL (it also happens with layers.acceleration.force-enabled=true). Desktop environment does not matter, in particular whether there is a compositor or not. It is also not specific to a recent version of the kernel, Xorg, NVIDIA driver, Firefox, as I have this issue since many months.

 

I have Googled this issue to no avail, so I'm desperate to find a workaround. I know WebRender is not enabled by default but it works otherwise perfectly fine with great performance.