Getting [NSEvent mouseLocation] in view coordination system

You should be using locationInWindow, not mouseLocation. The docs on locationInWindow show how to go from there:

NSPoint event_location = [theEvent locationInWindow];
NSPoint local_point = [self convertPoint:event_location fromView:nil];

Swift

override func mouseMoved(with event: NSEvent) {
    let mouseLocation = event.locationInWindow
    print(mouseLocation)
}