Active Window

An active window is the top-level window of the application with which the user is currently working. To allow the user to easily identify the active window, Windows 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. Only a top-level window can be an active window. When the user is working with a child window, Windows activates the top-level parent window associated with the child window.

Only one top-level window in the system is active at a time. 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. An application activates a top-level window by calling the SetActiveWindow function. A number of other functions can cause Windows to activate a different top-level window, including SetWindowPos, DeferWindowPos, SetWindowPlacement, and DestroyWindow. 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. An application uses the GetActiveWindow function to retrieve the handle of the active window.

When the activation changes from a top-level window of one application to the top-level window of another, Windows sends a WM_ACTIVATEAPP message to both applications, notifying them of the change. When the activation changes to a different top-level window in the same application, Windows sends both windows a WM_ACTIVATE message.

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