mouseReleaseEvent has never occur when I press on QStatusBar's size grip. Please see attached example for reproducing.

How to reproduce:
1) Build and run exampel
2) On status bar you will see the current state of mouse buttons
3) Please press (e.g. left button) on central widget.
4) On status bar you will see the Qt::LeftButton (it is right)
5) Please release mouse button now
6) On status bar you will see the Qt::NoButton (it is right also)
7) Now please press on QStatusBar's size grip.
8) On status bar you will see the Qt::LeftButton (it is right)
9) And now release the mouse button.
10) On status bar you will see the Qt::LeftButton (it is not right behaviour)

So, as you can see mouseReleaseEvent has never occur for size grip.