The GetCapture function retrieves the handle of the window (if any) that has captured the mouse. Only one window at a time can capture the mouse; this window receives mouse input whether or not the cursor is within its borders.
HWND GetCapture(VOID)
This function has no parameters.
The return value is the handle of the capture window associated with the current thread. If no window in the thread has captured the mouse, the return value is NULL.
A NULL return value does not mean no other thread or process in the system has captured the mouse; it just means the current thread has not captured the mouse.