Function fallback_class 

pub fn fallback_class(fallback: Fallback) -> &'static str

The class a run carries for what it does when it is tight.

A run always carries .run as well, so an unrecognised fallback – the enum is #[non_exhaustive] – lands as a plain nowrap row with the min-content floor still under it. That is the safe failure: every member in flow and none overlapped, which is the property, with only the rearrangement missing.