What does "replaced with addition of the dirty area" mean? The X
server only sends expose events when it thinks that a (part of) an X
window needs to be redrawn, for example when resizing.
I think it means "inform the X server that a region needs to be redrawn,
as if it had just been exposed". I don't know if X even supports this
idea, but I know it from Java:
<http://java.sun.com/j2se/1.5.0/docs/api/java/awt/Component.html#repaint%28int,%20int,%20int,%20int%29>.
That method arranges for a repaint to occur, but returns immediately; the
painting happens in response to the artificial expose-like event created.
(Such events may then be intelligently coalesced by the toolkit to paint
larger areas in one pass.)