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 IDirectInputDevice8::GetDeviceData or the IDirectInputDevice8::GetDeviceState 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.