This message is sent when the clipboard contains a data handle for the CF_OWNERDISPLAY format (that is, the clipboard owner should display the clipboard contents) and the clipboard-application window has changed size.
wParam
Identifies the clipboard-application window.
lParam
The lParam parameter is a global DDESHARE handle that when locked points to a RECT structure that specifies what part of the client area to paint.
A WM_SIZECLIPBOARD message is sent with a null rectangle (0,0,0,0) as the new size when the clipboard application is about to be destroyed or minimized. This permits the clipboard owner to free its display resources.
An application must use the GlobalLock function to lock the memory that contains the PAINTSTRUCT data structure. The application should unlock that memory by using the GlobalUnlock function before it returns.
GlobalLock, GlobalUnlock