Thumbnails still clickable, just strange picture in picture effect. Haven’t changed graphics cards or monitors or completed any updates recently, nor have I installed any new extensions.
Do you use an external display? What scale factor is your display using (SketchUp supports up to 150%)? Is your Nvidia driver up to date?
You have to check the high dpi setting in the properties panel (right click on the docked icon)
It’s only happening in 2021, though, not for 2020, so this does appear to be a product bug, and it has been logged with our engineers for fixing, but I don’t think it’s going to make it into a 2021 fix.