Hi, I tried to reproduce these issues on a Dell Laptop with NVIDIA GeForce GTX 1050 and a second monitor attached to it but without success, I tested various websites like 9gag, pinterest, sketchfab, twitch, netflix, youtube but mostly Reddit.
Scrolling, resizing windows, Zooming in and out for a few hours changing custom themes or using the Dark Reader add-on did not help me reproduce these issues.
I also left the Laptop on overnight with a lot of tabs open and a few games started in separate tabs and windows like https://www.kongregate.com/games/kodiqi/idle-breakout or Facebook Farmville but no glitches or issues occurred.
We also tested this issue with just the Laptop's display no external monitor attached and using the same websites and stress factors but we couldnt reproduce those glitches on our end.
Chances are those issues are caused by more than just one factor like WebRender, and maybe even with the same exact GPU's we still wouldnt be able to reproduce the issue. Maybe the users plug in and out their external monitors more often, or maybe they are using docking stations which might make their gpu's go haywire.
Please let me know if there is anything more we can do. In the meantime we will keep an eye out for these types of glitches.