Great! And thanks for providing those follow up details. That’s very interesting that CCleaner fixed the issue. That leads me to suspect that a different Qt5 DLL was being used by GPlates somehow (perhaps the older version Qt < 5.15.2, that had the mouse offset issue).
We place DLLs used by GPlates in the same directory as the GPlates EXE, so they should get found first (ie, before any system installed DLLs). Although there is one note mentioned here that might have caused your issue (prior to using CCleaner) - apparently before searching the application directory it does the following (which I hadn’t seen before)…
If a DLL with the same module name is already loaded in memory, the system uses the loaded DLL, no matter which directory it is in. The system does not search for the DLL).
…so perhaps some other application, using Qt5, was run before GPlates.
And regarding the green lines, that’s a good point (that it could be using the integrated GPU, except when using external monitor where it might be using dedicated GPU). I wonder if the integrated GPU drivers got updated (maybe it was the dedicated that got updated).