[css-flexbox] clarify whether collapsed flex items influence the flex container's intrinsic main size

[dholbert]

The "flex container intrinsic main sizes" spec-text (section 9.9.1) doesn't currently say anything special about excluding or special handling for collapsed flex items. It probably should.

I think they are intended to have zero contribution to the container's main size, based on a side-comment in the "Collapsed Items" section, section 4.4 -- that section says: "...collapsing or uncollapsing items may change the flex container’s main size", but never says precisely how. (That section does say that these items turn into zero-main-size "struts", but that happens after the container's intrinsic sizing.)

Probably https://drafts.csswg.org/css-flexbox-1/#intrinsic-main-sizes step 1 needs:

s/For each flex item/For each flex item (excluding collapsed flex items)/

...or something along those lines.

[dholbert]

actually, step 3 is the most important step to adjust: add each item’s flex base size to the product of [...]. We need to define things such that this added quantity ends up at 0, for collapsed flex items (or simply exclude collapsed flex items from the "add each item's..." operation here.)

[dholbert]

I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1692366 on fixing this in Firefox/Gecko (to exclude collapsed items from the container's main-axis intrinsic sizing)

[fantasai]

@dholbert Edits checked in, let us know if we missed anything?
Agenda+ to verify with the CSSWG.

[dholbert]

The edits look good to me. Thanks!

[css-meeting-bot]

The CSS Working Group just discussed Flexbox, and agreed to the following:

• RESOLVED: Accept edits

The full IRC log of that discussion

<fantasai> Topic: Flexbox
<TabAtkins> github: https://github.com//issues/5985
<fantasai> github: https://github.com//issues/5985
<fantasai> TabAtkins: just got approval from dholbert so probably easy
<chrishtr> I need to follow up some more on issue 5896, let's cover it another time.
<fantasai> TabAtkins: dholbert noticed that collaped flex items aren't explicitly excluded from intrinsic size calculation
<fantasai> TabAtkins: so we updated to explicitly skip those items
<fantasai> commits are https://github.com/w3c/csswg-drafts/commit/60ffc4058780d832d880a076fe02788f0cc6e8a7
<fantasai> TabAtkins: We made the fix, just wanted WG confirmation it's OK
<cbiesinger> I haven't read the text but your description sounds good
<fantasai> astearns: proposal to accept?
<fantasai> RESOLVED: Accept edits

[tabatkins]

Implicitly tested by https://github.com/web-platform-tests/wpt/blob/master/css/css-flexbox/flexbox-collapsed-item-horiz-001.html