An application sends this message to a multiple document interface (MDI) client window to instruct the client window to activate a different MDI child window. As the client window processes this message, it sends WM_MDIACTIVATE to the child window being deactivated and to the child window being activated.
wParam
When the application sends the WM_MDIACTIVATE message to its MDI client window, the wParam parameter contains the window handle of the MDI child window to be activated. When the client window sends the message to a child window, wParam contains the window handle of the child window being deactivated.
lParam
When the application sends the WM_MDIACTIVATE message to its MDI client window, the lParam parameter should be set to zero. When the client window sends the message to a child window, lParam is the window handle of the child window being activated.
MDI child windows are activated independently of the MDI frame window. When the frame becomes active, the child window that was last activated with the WM_MDIACTIVE message receives the WM_NCACTIVATE message to draw an active window frame and caption bar, but it does not receive another WM_MDIACTIVATE message.
WM_NCACTIVATE