This is a collection of notes on using floating subviews in a a scroll view within an AppKit Mac app. When you add a floating subview, it scrolls in sync with the document along one axis, while remaining fixed in position along the other axis, and it does so efficiently and with minimal code.

I wanted to publish these notes after adopting floating subviews earlier this year and finding some gaps in the documentation, some surprising behaviors, and not many good search results for my questions. (AppKit developers often run into this, especially for technologies which are so different from their UIKit equivalents.) The below notes are an attempt to capture, in one place, some key aspects of how floating subviews work and some things to watch out for.

Note that this will all make a lot more sense if you’re already familiar with NSScrollView; in particular, with how it works with NSClipView to implement scrolling over its document view (which is all flattened together in the much simpler UIKit model).


Among a TimeStory document’s subviews are some which float along an axis; for example, the time index at the top, which scrolls left and right with the document body but stays at the same place vertically.

Back during 2.2, I switched some of these views to start using floating subviews; previously, I had beeen using bounds-change notifications to directly synchronize their layouts, and this switch simplified the code and improved scrolling performance. Worth doing, but with a few gotchas waiting; see “Caveats”, below.