Window list extension causes flickering black borders around windows
It took a few days to figure this out but I am here reporting what I observed.
I am on Arch Linux running linux-lts 5.4.55, with Radeon RX580 Nitro+ 5GB using mesa 20.1.4, amdgpu enabled and correctly loading at boot. Gnome version is 3.36.4 and I am running a Wayland session. I have a HiDPI monitor DELL UltraSharp U2718 with native resolution 3849x2160 @ 60Hz, connection is DP on the GPU side and mDP on the monitor. Tested also with HDMI, same behavior observed.
When running at native resolution, enabling the fractional scaling by setting the factor to 125% or greater, I experienced flickering borders around windows. Similar borders were appearing around text in terminal and other applications. With some trials and errors, I figured out that it could have been an extension. Testing by disabling one by one the extensions I had loaded (six or seven, can give the list if needed), the behavior was gone by turning off the "window list" extension. Also, with scaling factor set to 100% the behavior does not show up.
I made a gif to exemplify what I am talking about. In the fist part of the clip I show that with no scaling the window gets drawn properly, then I turn the scaling on and the weird box appear: https://imgur.com/FUu3dmV
If you need more details I would be more than happy to report.