[flohofwoe]

Those are most likely windows which have been created without window chrome. The Win32 CreateWindowEx() function has style flags which allows all sorts of modifications down to "just give me a featureless rectangle to draw into". That's a really nice thing about the Win32 window system, it gives you a fully featured default window and then you can gradually override the defaults as needed.

[okanat]

I think they actually don't use that flag but use this function here: https://learn.microsoft.com/en-us/windows/desktop/api/Dwmapi...

The feature is from Windows Vista and it is still the basis of acyrilic effects on 11. The feature is explained in this page: https://learn.microsoft.com/en-us/windows/win32/dwm/customfr...