Qt::WA_TransparentForMouseEvents

[zaphod.b]

Our desktop app has an overlay over the main window with an "aperture" that is made transparent to mouse events by setting the Qt::WA_TransparentForMouseEvents attribute. This used to work well in an earlier version based on Qt 5.9.8.

We have migrated to Qt 6.2.3, and the event pass-through has stopped working on Linux (ubuntu 20.04) and MacOS, but continues to work on Windows.

I assume the attribute is ultimately handled by the window manager. Is this assumption correct?
Could the propagation of the attribute to the window manager possibly have changed between Qt 5.9.8 and Qt 6.2.3?

How can I even test this hypothesis? So far I can only confirm that mouse clicks are not consumed by the "underneath" widget as they should be (and are on Windows).

Are there other ways to achieve z-order event transparency for a widget?