Comment 169 for bug 1872001

I am seeing a very similar issue on my Lenovo T480s while using the Intel UHD Graphics 620 (rev 7).
A simple example is that YouTube videos in any browser (Chrome/Chromium/Firefox/etc.) while in the browser window play at 60fps but as soon as you use full screen it falls back to 30fps on the latest 5.4 kernels (-54,-53,-52). I did a quick test and booted a 4.15.0-123 kernel and that seemed to resolve the problem. Seems like a regression.