Mouse Capture

The system typically posts a mouse message to the window that contains the cursor hot spot at the time a mouse event occurs. An application can change this behavior by using the SetCapture function to route mouse messages to a specific window. The window receives all mouse messages until the application calls the ReleaseCapture function or specifies another capture window, or until the user clicks a window created by another thread.

Whenever the mouse capture changes, Windows sends a WM_CAPTURECHANGED message to the window that is losing the mouse capture. The lParam parameter of the message specifies the handle of the window that is gaining the mouse capture.

Only the foreground window can capture mouse input. When a background window attempts to capture mouse input, it receives messages only for mouse events that occur when the cursor hot spot is within the visible portion of the window.

Capturing mouse input is useful if a window must receive all mouse input, even when the cursor moves outside the window. For example, an application typically tracks the cursor position after a mouse "button down" event, following the cursor until a "button up" event occurs. If an application has not captured mouse input and the user releases the mouse button outside the window, the window does not receive the button up message.

A thread can use the GetCapture function to determine whether one of its windows has captured the mouse. If one of the thread's windows has captured the mouse, GetCapture retrieves the handle of the window.

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