WM_MDIGETACTIVEThe WM_MDIGETACTIVE message retrieves the multiple document interface (MDI) child window that is active, along with a flag indicating whether the child window is maximized.
This message has no parameters.
The return value is the handle of the active MDI child window in its low-order word. If the window is maximized, the high-order word is 1; otherwise, the high-order word is 0.