The following code works as expected: But do you know what “expected” means in this case? As a reader, you assume the author had an intention. You look for the mens auctoris and are an overall benevolent reader, I hope. Presupposing said intention, you may assume that it does something special if you put the call to items.removeAll() in a defer block.
NSTextView (and UITextView for that matter) have a defaultParagraphStyle attribute where you can set the text’s line height. That works swell – if you display text statically. Once the user can enter something, you can run into trouble: Update 2017-07: I posted a better version without paragraph style attributes that hooks into the NSLayoutManager delegate callbacks for a more consistent and speedy experience!
I wrote about using NSUndoManager + ReSwift with a Middleware. You could think of the resulting UndoMiddleware as some kind of observer of events that pass the ReSwift.Store and which puts the opposites of incoming actions on the undo stack. Why a Middleware? Because making an action undoable is a side effect.