Comment 9 for bug 1547864

Analogously in the case of GUI toolkits, one would usually act on a button release, but only if it was preceded by a button press. The app however could encounter a press without a release (mouse dragged off the button) or a release without a press (mouse dragged onto the button). In the former case the button is informed that the press has been cancelled (by the pointer leaving) before it was released. And in the latter case the button simply ignores a 'release' that had no press before it. So there's another precedent for button cancellation of sorts...