You will have to register to the notifications (NSWindowWillExitFullScreenNotification, NSWindowWillEnterFullScreenNotification) and adjust the frame of the button accordingly.
For example
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(windowWillEnterFullScreen:)
name:NSWindowWillEnterFullScreenNotification
object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(windowWillExitFullScreen:)
name:NSWindowWillExitFullScreenNotification
object:nil];
And later
- (void)windowWillEnterFullScreen:(NSNotification *)notification
{
// set your button frame to the left
}
- (void)windowWillExitFullScreen:(NSNotification *)notification
{
// set your button frame back to the centre
}
Hope this helps