[css-flexbox] Clarify whether unresolvable percentages prevent stretching?

[cbiesinger]

https://drafts.csswg.org/css-flexbox/#valdef-align-items-stretch

If the cross size property of the flex item computes to auto, and neither of the cross-axis margins are auto, the flex item is stretched

Combined with:
https://drafts.csswg.org/css2/visudet.html#the-height-property

the height of the containing block is not specified explicitly (i.e., it depends on content height), and this element is not absolutely positioned, the value computes to 'auto'.

So I just wanted to verify that the item should still stretch in a case like this:

<div style="display: flex;">
  <div style="height: 100%; background: green"></div>
  <div style="height: 200px;"></div>
<div>

Note that neither Chrome nor Firefox currently stretch the item.

@dholbert @fantasai @tabatkins @davidsgrogan

[Loirooriol]

An indefinite percentage only behaves as auto, but doesn't actually compute to auto. That's just legacy prose from css2. See https://drafts.csswg.org/css-sizing-3/#behave-auto

[davidsgrogan]

The Note in https://drafts.csswg.org/css-sizing-3/#behave-auto says

Note: Legacy spec prose defining layout behavior might explicitly refer to width/height having a computed value of auto as a condition; these should be interpreted as meaning behaves as auto, and reported to the CSSWG for updating.

Does that mean that https://drafts.csswg.org/css-flexbox/#valdef-align-items-stretch should change from

If the cross size property of the flex item computes to auto, and neither of the cross-axis margins are auto, the flex item is stretched

to

If the cross size property of the flex item behaves as auto, and neither of the cross-axis margins are auto, the flex item is stretched

Which would bring us back to the original question?

[Loirooriol]

I believe the flexbox prose is not legacy so it really means "computes to auto".

For example https://drafts.csswg.org/css-flexbox/#min-size-auto uses the new terminology: "percentages calculated against this size will behave as auto"

[cbiesinger]

Per #4525 indefinite percentages do not count as auto and thus should not stretch the item.