And, of course, that means it isn't interoperable ;-)
So even if it is defined somewhere implementers didn't find it...

https://drafts.csswg.org/css-multicol-1/#pseudo-algorithm

I think Firefox makes sense when you need fit-content:

https://jsfiddle.net/L03rqecj/2/

• Edge allocates 10 columns, then fill only three of them
• Firefox allocates as many columns as the container width allows, then fill only three of them
• Chrome allocates as many columns as the container width allows, then fill only two of them by wrapping the first two lines on the same column, but not the third one (wait, what)?

And it makes sense again when you need min-content:

https://jsfiddle.net/L03rqecj/3/

• Edge does as it does in the previous case (whaaaat?)
• Firefox collapses the whole thing to one column (of about 10px size)
• Chrome does like it did for preferred width (whaaaat?)

I have seen real interop issues due to this while digging some tables bugs.

We should sort this out ;-)