The QScroller class has a grabbedGesture() method, I think it'd be useful if QWidget also had a method allowing to determine whether a widget already subscribes to a particular gesture. I'm sure the reasons why QScroller::grabbedgesture() exist can be extended to many other QWidgets.

My own context: a lowlevel eventFilter implementation of a generic click-and-hold opens the contextmenu behaviour. Knowing when a widget already subscribes to the gesture would make this easier and safer.