Mouse wheel and trackpad scroll

[eligolding]

Add support for scrolling up when you hit the up arrow. Make sure you can’t scroll past the top of the page. It’s harder to stop scrolling past the bottom of the page; we will implement this in Chapter 5 Then bind the event, which triggers when you scroll with the mouse wheel.It will also trigger with touchpad gestures, if you don’t have a mouse. The associated event object has an event.delta value which tells you how far and in what direction to scroll. Unfortunately, Mac and Windows give the event.delta objects opposite sign and different scales, and on Linux, scrolling instead uses the and events. (The Tk manual has more information about this. It’s not easy to write cross-platform applications!)