[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?

(*) Added for clarification purposes.