1018449 - Firefox stretches flex items to container's cross-size, in some multi-line cases, when it shouldn't

Status: RESOLVED FIXED

(Core :: Layout, defect)

Description

[Daniel Holbert [:dholbert]]

Spec quote:
  As a special case for handling stretched flex items, if a
  single-line flex container has a definite cross size, the
  outer cross size of any flex items with align-self: stretch
  is the flex container’s inner cross size (clamped to the flex
  item’s min and max cross size) and is considered definite.
http://dev.w3.org/csswg/css-flexbox/#layout-algorithm

In vertical flex containers, we apply this here:

>1099   aFlexItem.ResolveStretchedCrossSize(aParentReflowState.ComputedWidth(),
>1100                                       aAxisTracker);
http://mxr.mozilla.org/mozilla-central/source/layout/generic/nsFlexContainerFrame.cpp?rev=8aa36dab2765#1099

HOWEVER, this code doesn't check whether we're single-line or not. It should. (And it shouldn't stretch the auto-size [to the container's cross size] at this point, if we're multi-line.)

I don't have a testcase yet, but I suspect it shouldn't be hard to construct one that we fail because of this.

(According to the linkification in the spec, "single-line" here is purely based on the "flex-wrap" property -- it does not include cases with flex-wrap:wrap w/ only enough content to fill a single line. That's good, because if it did include that scenario, we wouldn't know whether or not to apply this special-case, because at this point in the code, multi-line flexboxes don't necessarily know yet whether they'll occupy more than one line or not.)

Comment 1

[Daniel Holbert [:dholbert]]

Attachment: testcase 1

Comment 2

[Daniel Holbert [:dholbert]]

Attachment: fix v1

Comment 3

[Daniel Holbert [:dholbert]]

Try run: https://tbpl.mozilla.org/?tree=Try&rev=aa11b78fdfab

Comment 4

[Mats Palmgren (inactive)]

Comment on attachment 8435548 [details] [diff] [review]
fix v1

Makes sense, thanks for the detailed explanation (as always :-) )

Comment 5

[Daniel Holbert [:dholbert]]

Thanks! Landed: https://hg.mozilla.org/integration/mozilla-inbound/rev/6c2881dc6141

Comment 6

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/6c2881dc6141