The WM_SIZECLIPBOARD message is sent to the clipboard owner by a clipboard viewer window when the clipboard contains data in the CF_OWNERDISPLAY format and the clipboard viewer’s client area has changed size.
WM_SIZECLIPBOARD hwndViewer = (HWND) wParam; // handle of clipboard viewer window hglbRc = (HGLOBAL) lParam; // handle of RECT object
When the clipboard viewer window is about to be destroyed or resized, a WM_SIZECLIPBOARD message is sent with a null rectangle (0, 0, 0, 0) as the new size. This permits the clipboard owner to free its display resources.
The clipboard owner must use the GlobalLock function to lock the memory object that contains RECT. Before returning, the clipboard owner must unlock the object by using the GlobalUnlock function.