[css-grid-3] Should align-self/justify-self: stretch override explicit size?

[celestepan]

CSS Grid and CSS Flex both ignore align-self: stretch and align-items: stretch when an item has a specified size. Part of our motivations in this discussion: #10275 (comment) were to fill out the masonry layout visually, avoiding empty bottoms.

Generally, it makes sense to me that web authors would just expect the same behavior as CSS Flex and CSS Grid, and not specify item sizes when also applying align-items: stretch to them. The concern I have here is a case where authors want uniformly-sized items, but want the last row to fill out the page with stretch. If we ignore the explicit sizes, we would end up with:

while if we were to follow the explicit sizes we'd get something more like:

I think it makes sense to respect explicit sizes -- if an author is specifying explicit sizes, it's likely that they want to maintain that specific size of the item throughout the layout. It would also be the most expected behavior, in parity with CSS Grid and CSS Flex.

[celestepan]

cc @alisonmaher @mhochk @KurtCattiSchmidt @yanlingwang23

[Loirooriol]

stretch self-alignment affects automatic sizes, I think it would be very confusing to change that.

Even with an auto size, can stretch self-alignment even do what you want? I haven't been following the details, but if that's possible, you should be able to set a stretch size instead.

I'm not sure how you plan to select the elements at the end, though.

[celestepan]

@Loirooriol I was referring to explicitly sized items, not auto-sized ones! I agree that auto-sized items should be affected.

Also, if I'm understanding the discussion correctly, the plan had been to only apply stacking-axis alignment to items above a span, where the placement of the span caused excess space, or the last item in a track. So I wouldn't be selecting the items explicitly, it would be something more like the container having align-items: stretch, such that the last items end up being the only ones having the space to stretch.