[css-multicol][css-align] row-gap should apply to multi-column containers too

[MatsPalmgren]

CSS Box Alignment says:

multi-column containers [...] row-gap does not currently apply.
https://drafts.csswg.org/css-align-3/#gap-multicol

I think it should apply to multi-column containers too, specifically between column-span elements and the set of columns above/below. I'm working on a prototype implementation of gap decorations in Gecko, which adds row-rule properties, which will render between the "rows" created by spanners. So it's reasonable to change row-gap so that it applies to multi-column containers too. (I'll file a separate issue on the gap decorations proposal.)

Like for Grid and Flexbox gaps, the row-gap should be applied between margin-boxes, and it doesn't collapse or merge with any margins. It creates space in addition to any margins on the spanner.

Here's an example multi-column container with row-gap and (purple) row-rules:

[rachelandrew]

I'm hoping to spec block direction overflow for multicol containers in level 2 of the spec, in which case the row-gap (and any decoration) would be between the new sets of column boxes in the block dimension.

Given that it is already possible to make space around a spanner using margins, my initial thought is that we should maintain the gap property for this use rather than using it for spanners.

[MatsPalmgren]

I don't see a conflict there (assuming I'm imagining how your "block direction overflow" thing would work correctly). I tend to think row-gap/rules should appear in all "block axis divisions" in a column layout. I elaborate a bit more in the draft spec about how it would work for column-span: <integer> for example.