Is this expected on a mac? If so is there a way that I can set the focus back to this main window and get the current position of the mouse?
Funny you'd mention this just now since we were just discussing this over at #lwjgl a few days ago.
Now the issue here is that Mac's use a Java AWT Frame to create their LWJGL Display window. The problem here seems to be with how the AWT Frame works on mac, if it doesn't have focus it doesn't receive any mouse events. However an AWT Frame on windows and linux works just fine and receives mouse events even when Frame is not focused.
I'm not really sure what the LWJGL team can do here since this appears to be an issue with AWT on mac.
A possible solution would be to write a native window system for LWJGL like linux and windows have, however this is no small task and would require an almost complete rewrite of the mac support for LWJGL. If anyones up for this task feel free to attempt it, this would also be very useful in many other respects too.