This message indicates that the window is being sized, or that a WinLockWindowUpdate has been issued for the window or one of its parent windows. Direct drawing to the window should be suspended.

param1
     VOID   mp1         /*  Reserved value. */

param2
     VOID   mp2         /*  Reserved value. */

returns
     ULONG  ulReserved  /*  Reserved value, should be 0. */