An issue raised in October 2013 on www-style:

An issue come up today in a discussion with a seasoned multicol
implementer: does overflow content influence column height?

Consider a multicol element with two columns, and column breaks after
each paragraph:

article { columns: 2; background: yellow }
p { break-after: column }

Then you pour three p elements into the article: two one-liners and then one long paragraph (say, 10 lines). What's the resulting height of the article? 1 or 10 lines (disregarding padding for a moment).

I can see two alternatives:

(1) the multicol element is made high enough to fit all columns. In this case, there will be three columns: two with one line each, and a third column -- outside the multicol element -- with 10 lines. The yellow box will be high enough to fit the third column, even if it shown outside the multicol element.

(2) the multicol element is made high enough to fit all columns that end up inside the box. So there will be two columns with one line of text each inside the multicol element, and then there will be 10 overflow columns outside.

I think I have a slight preference for (2).

Implementations vary, and it's hard to test as not all support explicit column breaks.

In comments 1, 2 on this issue @astearns and @tabatkins seem to agree on 2.