If you press something (like a combobox or an icon) which (immediately or eventually) results in a popup being opened, you ought to be able to mouse over or move your finger over to items in the popup and release, without lifting the button or the finger first. E.g. maybe the popup has more icons with TapHandlers intended to detect the release.

One way is imperatively (when the popup opens, ask the original grabber to give up its grab and somehow re-deliver via QQuickWindowPrivate::deliverPressOrReleaseEvent() ?) but does it have to be imperative?

Dealing with event forwarding to popup windows is related but (mostly?) out of scope of this task, because that will probably depend on some work in QtGui.