The frame window procedure processes this message in the following manner:

• The frame window sends this message to the client in response to the frame WM_PAINT message, with the presentation-space handle of the frame window (obtained from WinBeginPaint).

• If the client window returns TRUE, the frame window procedure erases the rectangle of the frame window covered by the client window, by filling it with the system color SCLR_WINDOW.

• If the client window returns FALSE, no action is taken. This is the default behavior, as WinDefWindowProc returns FALSE if passed this message.

• Also, the client window can use the presentation-space handle passed in this message to selectively erase parts of the screen. If the client window processes the message in this way, FALSE should be returned to avoid the erasure being done automatically by the frame window procedure.

  It should be noted again that the presentation space is not a client window presentation space; it is a presentation space for the frame window returned by WinBeginPaint, that is, a cached presentation space in frame (not client) window coordinates, clipped to the area of the frame that needs to be updated (possibly including areas outside the client window).