Holy cow, I wrote a book!
Or at least, that's what people would claim.
Actually, Windows is painting your window with your class
background brush. Since most people use COLOR_WINDOW and since
COLOR_WINDOW is white in most color schemes, the end result is
a flash of white.
Why paint the window white? Why not just leave it alone?
Well, that's what it used to do, but the result was that
the previous contents of the screen would be shown where
the window "would be". So suppose you were looking at
Explorer, and then you restored a program that stopped
responding. Inside the program's main window would be...
a picture of Explorer. And then people would try to
double-click on what they thought was Explorer but was
really a hung program.
In Windows XP, the behavior for a window that has stopped
painting is different. Now, the system captures the pixels
of the unresponsive window and just redraws those pixels
if the window is unable to draw anything itself.
Note, however, that if the system can't capture all of
the pixels - say because the window was partially covered -
then the parts that it couldn't get are filled in with
the class brush.