
Suppose you have an idle app with a realized and mapped Window that contains
Xlib graphics.  A button widget, when pressed, will cause a new item
to be drawn in the Window.  This action clearly should not call XCopyArea() 
(or equiv) directly; instead, it should register the existence of the new
item in a memory structure and let the expose event handler take care
of rendering the image because at that time it is guaranteed that the
Window is mapped.