[css-align][css-grid] Synthesizing baselines for elements orthogonal to the baseline-axis

[javifernandez]

The CSS Box Alignment spec states the following:

https://drafts.csswg.org/css-align-3/#baseline-terms

For this purpose, boxes whose block flow direction is parallel to the alignment context’s axis are treated as having a block flow direction orthogonal to the alignment context’s axis and directed end-ward relative to the box that establishes the alignment context, and their baselines are synthesized accordingly, exactly as for replaced elements and other boxes that have no natural baseline.

When applied to actual orthogonal items (orthogonal flow respecting the grid container), the text above matches perfectly to what the CSS Writing Modes spec states about synthesizing baselines:

https://drafts.csswg.org/css-writing-modes-3/#replaced-baselines

The alphabetic baseline is assumed to be at the under margin edge.

Well, the CSS Box Alignment spec clarifies that the edges used to synthesize the baseline depend on the formatting context:

https://drafts.csswg.org/css-align-3/#synthesize-baselines

Note: The edges used to synthesize baselines from a box depend on their formatting context: inline-level boxes synthesize from their margin edges [CSS-INLINE-3], table cells synthesize from their content edges [CSS2], and grid and flex items synthesize from their border edges [CSS-GRID-1] [CSS-FLEXBOX-1].

Hence, for grid items it seems that under border edge must be used to synthesize elements orthogonal to the baseline-axis (or equivalent, parallel to the alignment context’s axis). As I said, when applied to actual orthogonal boxes this under is resolved as bottom or left depending on the grid item's writing-mode. This makes a lot of sense, so far.
https://drafts.csswg.org/css-writing-modes-3/#logical-to-physical

But the spec states that we should treat in the same way parallel elements when they are orthogonal to the baseline-axis. Hence, that would imply that we will have to resolve the under logical direction assuming those elements are "orthogonal" to the grid container, instead of using strictly their writing mode. Is this assumption right ?

I'm attaching a picture of what I've implemented, so far.

[javifernandez]

Please, @tabatkins @fantasai could you take a look at this issue ? Not that it's specially urgent, but I m afraid it gets lost, since it's not even tagged yet. Thanks.

[fantasai]

Hi @javifernandez , you lost me in the last paragraph. Can you give an example in code, please? :)

[javifernandez]

<style>
    .grid { display: grid; grid-auto-flow: row; }
    .contentStart { align-content: start; justify-content: start; }
    .justifyItemsBaseline { justify-items: baseline; }
    .alignItemsStart { align-items: start; }
 </style>
 <div class="grid contentStart justifyItemsBaseline alignItemsStart">
   <div>A</div><div>A</div><div>A</div><div>A</div>
 </div>

Only the row-axis (justify-self) applies as baseline-axis, which determines we must select items which
share a column-like alignment context (the first and only column, in this case); the four items share such first column context.

All items have a block flow direction parallel to the alignment context's axis, hence we must treat them as:

having a block flow direction orthogonal to the alignment context’s axis and directed end-ward relative to the box that establishes the alignment context, ...

As the CSS Sizing spec states, we must synthesized the items' baseline using their under border edge. If we use the table defined in the CSS Writing Modes spec, we should map under to bottom because the items are horizontal. However, that doesn't make sense because the baseline-axis is the row axis, in this case, the horizontal axis.

So, I assumed that we must treat those items as orthogonal to the grid container, hence vertical, so that we map under to left

I wonder whether my assumption is correct, and in that case, whether it's properly described in the specs.

[fantasai]

You're right, it's not properly described in the specs. We've attempted to fix that. Let us know if it seems okay now? :)

[javifernandez]

Good, thanks fr clarifying this in the specs.