[jchb]

> so it can't gracefully scale layouts down from desktop to mobile sizes (when you have a 3-column layout you have lots of control over column widths, but no way of turning them into 3 rows instead).

On Apple platforms (UIKit on iOS) handling such layout changes is done with another mechanism on top of auto layout. Something they call "Size Classes". You get a callback saying eg. "view has changed to Regular width, Compact height" and you switch to a another set of auto-layout constraints + animate as desired. This size class system is used for stuff like handling rotation on the iPhone and transitioning to split-screen mode on iPad.