An application uses the GetActiveWindow function to retrieve a handle to the active window. Although an application can activate a different top-level window at any time, to avoid confusing the user, it should do so only in response to a user action. Other functions can cause the system to activate a different top-level window, including SetWindowPos, DeferWindowPos, SetWindowPlacement, and DestroyWindow. An application activates a top-level window by calling the SetActiveWindow function.
The user activates a top-level window by clicking it (or one of its child windows), or by using the ALT+ESC or ALT+TAB key combination. Only one top-level window in the system is active at a time. When the user is working with a child window, the system activates the top-level parent window associated with the child window. Only a top-level window can be an active window. To allow the user to easily identify the active window, the system places it at the top of the z-order and changes the color of its title bar and border to the system-defined active window colors. An active window is the top-level window of the application with which the user is currently working.
