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Window Regions
In addition to the update region, every window has a visible region that defines the window portion visible to the user. The system changes the
visible region for the window whenever the window changes size or whenever
another window is moved such that it obscures or exposes a portion of the window.
Applications cannot change the visible region directly, but Windows automatically
uses the visible region to create the clipping region for any display DC
retrieved for the window.
The clipping region determines where the system permits drawing. When the application retrieves a
display DC using the BeginPaint, GetDC, or GetDCEx function, the system sets the clipping region for the DC to the intersection
of the visible region and the update region. Applications can change the
clipping region by using functions such as SetWindowRgn, SelectClipPath and SelectClipRgn, to further limit drawing to a particular portion of the update area.
The WS_CLIPCHILDREN and WS_CLIPSIBLINGS styles further specify how Windows
calculates the visible region for a window. If a window has one or both of these
styles, the visible region excludes any child window or sibling windows (windows
having the same parent window). Therefore, drawing that would otherwise
intrude in these windows will always be clipped.
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