1328792 - Improve compatibility with Chrome/Safari, on behavior of abspos children inside of -webkit-box

Status: NEW

(Core :: Layout, defect, P3)

Description

[Daniel Holbert [:dholbert]]

I'm not entirely sure how abspos content is positioned inside of -webkit-box, in Chrome & Safari. But it doesn't work the same way as modern flexbox (before or after bug 874718).

When we introduced support for -webkit-box, we just made it use a modern flexbox under the hood, with the same behavior of abspos children. This has proved to be "good enough" - I don't believe we got any reports of sites that were broken due to mispositioned abspos content inside of a -webkit-box.

We did get reports after we reworked the way abspos content is positioned in modern flexbox (and hence in -webkit-box), though -- bug 1320484.

So for now (for simplicity and to avoid messing something that's worked decently well so far), I'm planning on reverting -webkit-box to the older abspos behavior (wrapping placeholders in anonymous flex items) over in bug 1320484.  And I'm filing this bug as a followup to cover known edge cases where that behavior doesn't match Chrome & Safari on abspos positioning inside of -webkit-box.  (These cases haven't yet been important for compatibility, so I won't be prioritizing this bug here just yet.)

Comment 1

[Daniel Holbert [:dholbert]]

Attachment: testcase 1: baseline alignment of -webkit-inline-box with abspos child

Here's the first testcase.  The expected behavior is that the underlines should all line up, on each line here.  This was broken in Firefox 50 (both bordered areas had higher underlines than the rest of their line), but we changed to the expected behavoir as part of modernizing flexbox abspos behavior.  But I'm rolling that change back for -webkit-inline-box in bug 1320484, which means the first box (the orange box) in this testcase will have go back to having a misaligned testcase.

(As outlined in comment 0, I'm not too concerned about this right now, since we're been shipping with this behavior and we're not aware of any compat issues associated with it.)