@alex_malyu Well, it actually not. At least in this case (I don't know if this depends on your operating system, or on the type of widget or something like that)
If I comment my update() call, paintEvent is never called, no matter how much I move the mouse over it.

It depends if it's a Qt Quick application or not. If you are using widgets or doing your own custom OpenGL rendering then there will be no problem. The issue with Qt Quick is that it has its own animation system which gets driven by the default 16.66 ms on platforms where it fails to query the real refresh rate from the system. Run with QSG_INFO=1 to verify the actual value that is in use. (NB Qt 5.5 has some improvements in this area - https://bugreports.qt.io/browse/QTBUG-44971 - so it is now able to retrive the correct refresh rate on embedded platforms too)