I think this is very major issue with the way how QT handles events on the mac. Steps to reproduce:

• Create QMainWindow
• Intercept event method
• Put console messages for NonClientAreaMouseButtonPress and NonClientAreaMouseButtonRelease

Mouse down on window title will generate NonClientAreaMouseButtonPress.
Mouse up on window title will generate NonClientAreaMouseButtonRelease.

Mouse down on one of the buttons in title bar (such as close or minimize) will generate NonClientAreaMouseButtonPress.
Move out the mouse pointer from the button and release it.
NonClientAreaMouseButtonRelease - is not generated. As a result - child widgets do not receive mouse messages anymore, since mouse was captured but not released.