When QGuiApplicationPrivate::processMouseEvent() is called with a MouseEvent where globalPos contains NaN, it will set positionChanged to true even if it is exactly the same position. If e->enhancedMouseEvent() is set and the event is not a NonClientAreaMouseMove or MouseMove event, it will recursively call QGuiApplicationPrivate::processMouseEvent(). (Normally there would only be one recursive call, but because NaN != NaN the code does not notice that it already moved the lastCursorPosition.) This endless recursion will cause a stack overflow.

Obviously there  should not be any NaN in globalPos, but e.g. the evdev touch code produces this sometimes. (I'll file another bug about that.)