[css-flexbox-1] Clarify how indefinite percentage values are handled in the cross size of flex items #12018
Description
The handling of height: 100% on a flex item, when the cross size of the flex container is indefinite, does not allow for stretching (see #4312 (comment) and #4525 (comment)).
This behavior is quite confusing (see #12012 (comment)). In fact, it's not clear what the expected result of height: 100% should be in this case (see #12012 (comment)).
The specification should state the behavior explicitly.