When you have set the cooperative level to DISCL_FOREGROUND and the focus switches to another application (or even to the menu in your own application), Microsoft® Windows® might force your application to unacquire the mouse. For this reason, you should check for the DIERR_INPUTLOST return value from the DirectInputDevice8.GetDeviceData or the DirectInputDevice8.GetDeviceStateMouse method, and attempt to reacquire the mouse if necessary. (See Acquiring Devices.)

Note  You should not attempt to reacquire the mouse on getting a DIERR_NOTACQUIRED error. If you do, you could get caught in an infinite loop: acquisition would again fail, you would get another DIERR_NOTACQUIRED error, and so on.