Tk Source Code

To Apple's Window manager a Tk window consists of one NSWindow containing one
NSView.  Tk is responsible for drawing its widgets inside the View. When you
move your mouse inside the View, Apple asks the View to redraw a damage region,
by calling the drawRect method of the view.  As I understand it, what happens is
that Apple passes the bounding rectangle of the View to its drawRect method.  So
Tk responds by redrawing everything that meets that rectangle, i.e. by redrawing
everything inside the window.  If Apple were to ask for updates only in a small
region containing the old pointer image, then maybe we could avoid sending expose
events for the children which do not meet that small damage region.

It is true that I have not spent any time trying to do this sort of optimization,
since other things were more urgent.  Maybe we could do better. If you can figure 
out how to get more detailed damage information from Apple, please let us know!