This little demo shows to make a fixed width (fully expanded or collapsed, no in-between) sidebar in an OS X app using NSSplitViewController. Expanding or collapsing can be done programmatically (with a button) or by the user sliding the divider 10 points to the right or left. Expanding or collapsing has the same nice animation in either case.
The "reuseable classes" are FixedSidebarSplitView and FixedSidebarSplitViewController, although you'll also need the -removeWidthConstraints method from NSView+SSYAutoLayout.
Initially, the sidebar view gets a width of 150 from a constraint in Interface Builder. This constraint is removed and replaced the first time that you enter a value into the Fixed Width field in the user interface.
This version requires OS X 10.11 or greater, and is a little tricky because the window is in a nib. According to the NSSplitViewController documentation, you can add views to split views "before calling super in -viewDidLoad, as in this demo. But it does not work in 10.10 because apparently there was a bug in 10.10: you could not add views at any time.
This restriction could be removed if you used a storyboard instead of a nib. This is because, in Interface Builder, when editing a storyboard but not when editing a xib, NSSplitViewController, with NSSplitViewItems, are available in the Object Library.