'mousescroll' was not exact because the mouse is also used to scroll with the scrollbar.  'handlescroll' is worse.  'wheelscroll' seems awkward. 'scrollwheel' (scroll with the mouse wheel) is specific.  At least in idlelib, event handlers are routinely called something_event, so use 'wheel_event'.  Pressing the wheel is a Button-3 event (there used to be 3-button mice before wheeels) and a handler for that would be 'button3_event' or 'button3_press_event'.