Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!apple!netcomsv!wolf
From: wolf@netcom.COM (Buckskin Tech.)
Newsgroups: comp.windows.ms.programmer
Subject: Re: Does a window know when it is being overwritten?
Message-ID: <1991May3.174333.6866@netcom.COM>
Date: 3 May 91 17:43:33 GMT
References: <59847@siemens.siemens.com>
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<in response to a question about being notified when a window is about to
be overwritten>

Yes, it's true that it's possible for the window to be overwritten whenever
you lose focus.  Therefore, it's a good idea to snapshot any dislay data just
before you lose focus.  As for menus, there is a message (WM_INITMENUPOPUP)
that is sent immediately before a menu is displayed.  You can try using a
WM_MEASUREITEM for each item in the menu about to be displayed to determine
the area that will be obscured.  

You're really only allowed to write to your entire client area if you use the
display context you get from Windows.  The DCs you can get all go through 
the installed video driver.  This driver expects to be aware of any and all
changes being made to the display, and have veto power over every change.
For hardware display updates, you'll probably need to have a special video
driver that can accomodate the display from the coprocessor.  There are
some real-time video cards for Windows that use this technique, I believe.

 - Wolf