When using a QGLWidget inside a mainwindow, which also has
dockwidgets or a statusbar, than the MouseMoveEvent is not always arriving.
It is only arriving when entering the QGLWidget with the mouse not passing the dockwidgets or the
status. The behaviour can be easily reproduced with the attached example.

The attached example is a simple QGLWiget with a statusbar using the mainwindow class. The bug occurs on OSX (Mountain Lion)
using Qt 5.0.1 or Qt 5.1 alpha. The bug only appears when using the mainwindow-class, without the program works fine.
This can also be tested with the example.