QApplication::mouseButtons() not updated when window gets focus

The attached test case displays the current state of the left and right mouse buttons as reported by QApplication::mouseButtons() (updated every 100 ms).

Incorrect behavior can be found in Qt 4.8.5 and a recent checkout of the Qt5 'dev' branch (be sure to add "QT += widgets" to the auto-generated .pro file):

(a) Right-click the label, such that a context menu appears. Then close the menu by left-clicking another application's window. Note that the "pressed" state for the left button is enabled on mouse press, but does not change any more after that because the test app has lost focus before the button is released. The bad thing about this is that the test app may re-gain the focus via the keyboard and then still maintain the incorrect mouse button state.

(b) Drag anything from another application and hover the test app's window. Note that the mouse button state is not updated.

This might or might not be related to https://bugreports.qt-project.org/browse/QTBUG-16635

These issues were reported for Dolphin:

https://bugs.kde.org/show_bug.cgi?id=271325
https://bugs.kde.org/show_bug.cgi?id=322644