I've attempted to fix this today but seems that [NSWindow setAcceptsMouseMovedEvents:YES] isn't going to be enough to fix this.
It seems that Cocoa app's in general can't get MouseMoved Events if the window isn't focused.
Apple did however address this problem by adding a new api called NSTrackingArea. This allows one to get MouseMovedEvents on non focused windows. The problem though is that NSTrackingArea requires OS X 10.5+ but can be added in a backward compatible way (i.e. won't do anything on OS X 10.3 and 10.4).
Need to spend more time to integrate NSTrackingArea into the code so will probably look into it later, hopefully sometime before LWJGL's 2.8 release but in the meantime ppl are free to also look into the problem and submit patches

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