The issue is on the native Cocoa/Objective-C side unfortunately. I did have a look however haven't been able to find a fix for it yet, its possible that it is a bug on Apple's side.
Currently its possible to implement a partial fix that would fix the behaviour for Java applications that are using Display.setParent() but would still leave the same stretching behaviour for applets, so waiting to find a proper fix.
The issue is basically that we are resizing a PBuffer. However the contents of the OpenGL context that is attached to the PBuffer remains the same size as when it was first created. It just stretches to any new Pbuffer size instead of also resizing itself. Likely some OpenGL setting is being overlooked or maybe something is broken somewhere in Apples new java api, no really sure atm.