10.9 Exposure Events
The X protocol does not guarantee to preserve the contents of window
the windows are obscured or reconfigured.
Some implementations may preserve the contents of windows.
Other implementations are free to destroy the contents of windows
X expects client applications to assume the responsibility for
restoring the contents of an exposed window region.
(An exposed window region describes a formerly obscured window whose
region becomes visible.)
Therefore, the X server sends
events describing the window and the region of the window that has been exposed.
A naive client application usually redraws the entire window.
A more sophisticated client application redraws only the exposed region.
Christophe Tronche, email@example.com