[css-flexbox-1] Clarify fit-content behavior when the available space in a given axis is indefinite

[gitspeaks]

According to CSS Flexible Box Layout Module, Section 9.4(7):

Determine the hypothetical cross size of each item by performing layout with the used main size and the [cross-axis*] available space, treating [flex item cross-size*] auto as fit-content.

However, according to the CSS Box Sizing Module Level 3, Section 2.1, the fit-content size in a given axis depends on the available space being definite:

If the available space in a given axis is definite, equal to clamp(min-content size, stretch-fit size, max-content size) (i.e. max(min-content size, min(max-content size, stretch-fit size)))

In particular, stretch-fit size is:

Undefined if the available space is indefinite.

and

Calculations involving this size need to specify a fallback behavior for when the available space is indefinite if that happens to be possible.

This raises a question: How should the fit-content value be determined for a flex item in situations where the [cross-axis*] available space is indefinite?

Consider the following scenario:

• Writing Mode: horizontal-tb
• Flex Direction: row
• Flex Container and Flex Items: Both have height: auto.

How should the fit-content size for the cross axis (height) of the flex item be determined?

Note: This step takes place after Section 9.7

(*) Added for clarification purposes.

[Loirooriol]

That isn't specific to flexbox, you removed it but the quote even says "as if it were an in-flow block-level box".

https://drafts.csswg.org/css-sizing-3/#fit-content-size

When sizing under a min-content constraint, equal to the min-content size. Otherwise, equal to the max-content size in that axis.

Also see #10721

[gitspeaks]

That isn't specific to flexbox, you removed it but the quote even says "as if it were an in-flow block-level box".

I didn't remove anything. I mistakenly included a link to the draft instead of the published version, where it clearly states:

Determine the hypothetical cross size of each item by performing layout with the used main size and the available space, treating auto as fit-content.

This is how the specification should be read as published.

[gitspeaks]

When sizing under a min-content constraint, equal to the min-content size. Otherwise, equal to the max-content size in that axis.

Just to confirm I understand correctly, the process is:

Treat the flex item as a block-level box in flow layout, where the available space in the main direction is determined by the previously calculated main size. Since the flex item's cross size is set to auto, we use fit-content and because the flex container has height: auto which does not impose a min-content constraint , the fit-content will be determined using max-content sizing in the cross direction — specifically, the max-content block size in case the cross direction is block.

Is that correct?

[Loirooriol]

where the available space in the main direction is determined by the previously calculated main size

The calculated main size is used as the main-axis size of the item, not as the available space in the main-axis.

[gitspeaks]

The calculated main size is used as the main-axis size of the item, not as the available space in the main-axis.

My intention was: 'available space' for the block-level box in a flow layout.

For horizontal-tb / row, read:

Treat the flex item as a block-level box in flow layout, where the available space in the inline direction (for the block-level box in flow layout) was determined by the previously calculated inline size of that box when it was considered a 'flex item.'