In #5335 we resolved that, when applying to a multi-column container, text-box-trim applies to every column; and in #11038 we clarified that text-box-trim applied to an ancestor also propagates accordingly.

However, we didn't discuss what happens if there's a column spanner. Technically there are column boxes both before and after the spanner, not a single column split by a spanner.

I imagine that if you want to trim in order to position the multi-column container, you probably don't want to affect its internal layout; so I suggest we only target the column boxes in the first/last row of column boxes--or the spanner, if that's the first/last item in a column...