Solved it by creating a derivative of the CustomQPushButton class definition! Thanks!

@legitnameyo This is not how it works. You will need to subclass QPushButton and override http://doc.qt.io/qt-5/qwidget.html#mousePressEvent
You can find an example here: http://doc.qt.io/qt-5/qtwidgets-widgets-scribble-example.html

@m_jero I don't know why nobody answers this question. Maybe because you didn't supply any code snippets as recommended. Anyway, I suppose you used the exec() function to show the qmenu. Try to use popup() instead and see if it works.

Alright, really strange, but just pulled mouse out tried touch pad. Guess what, just acted normal, put mouse back in no problem either. Pulled the mouse out earlier but didn't work then, don't know what changed but the problem is solved. (By the way, it was a cheap mouse with only 4 buttons :))

Thanks to both of you for your help and fast response.