Reproduction:

Create a MainWindow, override its mousePressEvent (...) method and set a breakpoint there (or a print to stdout). Set a fixed size to it,  so the lower right corner will not have any resizing grip showing. Place your mouse cursor on the lower right corner of the MainWindow. Press and hold any mouse button.

Expected:

Your stdout should print "mouse pressed"

Actually Happening:

Nothing is printed, the "mouse pressed" will however show up when you release the mouse. 

Conclusion:

This only happens on MacOs. When you place your cursor anywhere else on the Widget, the mouse press event happens exactly when you press the mouse (as expected).

See attached minimal reproduction example.