https://drafts.csswg.org/css-flexbox/#min-size-auto

This issue stems from an interop problem discussed in web-platform-tests/wpt@21a7c47 .

In the example below, the spec dictates a final width of the flex item of 50px. But when the same element is a block box it has width 100px. Should they be different?

<div style="display: flex;">
  <!-- Chrome and spec say final width is 50px. Firefox says 100px. -->
  <div style="background: blue; height: 100px; aspect-ratio: 1/2; ">
    <div style="width: 100px;"></div>
  </div>
</div>

<!-- Everyone agrees final width is 100px -->
<div style="background: blue; height: 100px; aspect-ratio: 1/2; ">
  <div style="width: 100px;"></div>
</div>

https://jsfiddle.net/dgrogan/2734sdfr/

Of course a flex item can have a different final size than an otherwise-identical block box because the whole point of a flex container is that it flexes the items. But this item is not flexed, and it's still a different size. That seems weird.

In more detail, the difference in this case is a result of aspect-ratio's automatic minimum size being more aggressive than flexbox's. Giving flex a weaker set of constraints makes sense, because we want to give flexbox the power to flex its items. But it is puzzlement-inducing that putting a no-op flex container around an element changes the element's size.

Possible solution if we want the boxes to be the same width whether they are block boxes or unflexed flex items (i.e. Firefox's behavior today):

1. change content-size suggestion to be min-intrinsic[1] instead of min-content (@bfgeek previously floated this idea In [css-sizing-4] Should aspect-ratio affect the intrinsic size? #5032 (comment))
2. Then we'd also have to change specified size suggestion to only account for computed main size properties, not preferred sizes that come from the aspect-ratio. As is, specified size suggestion is 50px, so even we did (1) and only (1), the final width of the item would still be 50px.

Or, do whatever Firefox is doing, which is probably not those.

But, if giving the blue box different sizes in the two layout modes doesn't strike anyone else as weird, then maybe we just close this issue and discard the idea of making content-size suggestion = min-intrinsic. Even if we do that, we still need to determine which of Firefox and Chrome behavior is correct.

[1] or whatever the name is, vis-a-vis #5305