Issue with WA_NoSystemBackground on MacOS

Use case is a complex widget that has multiple small details regions that change rapidly. Detail regions need to be repainted without redrawing the complex background.

Users approach:

"

My approach was to define an 'unlikely rectangle' and call update() with this rectangle. In the paint event I compare the QPaintEvent::rect to the unlikely rect to decide whether it's a detail repaint or a full repaint.

The approach works as expected on Windows and linux, but on MacOS the unlikely rect is always filled with white even though the WA_NoSystemBackground attribute it set.

Is this a platform limitation?

"

Qt Support tested the example on Windows and MacOs and it does behave differently.

Example project has been attached.