OS X "Notes" and "Mail" app implement NSSplitView in different fashion.
This sample app contains both implementation that simulate both behavior.
Check these 2 source files for detail of each implementation:
- DelegationSplitViewWindowController - NSSplitViewDelegate implementation that simulate "Mail" app
- AutoLayoutSplitViewWindowController - auto layout NSSplitView implementation that simulate "Notes" app
Please check the wiki on why the classes are implemented the way they are.