Rounded frame border on macOS

[weisJ]

When using decorations on macOS the frame border isn’t visible on the corners as it is painted as a rectangle. Instead we should either paint it as a rounded rectangle or not at all.
see #304 for an example

[anticleiades]

I would opt for not painting frame borders at all as a temporary workaround.

[weisJ]

I agree. The same issue occures on Windows 11.