Continuing down the rabbit hole, I stumbled upon this article by Nicole Sullivan that describes the applications of a little known W3C spec:
When the overflow property is set, a new block formatting context is created. What does that mean? It changes the way the block interacts with floats. It no longer wraps around floats, but rather becomes narrower.
In case you can’t tell, I’m still trying to wrap my head around inuit.css. More on that later.