The documentation _also_ says:

> Uses the `NSIntegralRectWithOptions` function to produce a backing store pixel aligned rectangle from the given input rectangle in window coordinates.

I'm pretty sure I recall running into a bug where it accepted window coordinates. There's an easy way to find out, though – we just need a unit test. :P 

> And if it doesn't have a window, I'm guessing `-backingAlignedRect:options:` wouldn't work. Maybe worth adding a comment about that?

There should either be a fallback mechanism or an assertion. In either case the behavior should be reflected in the documentation.