[crbug 606208] Flexbox and absolute children's painting order

[Dennis Schubert]

Hi everyone!

Two days ago, I added a WPT[0] to cover the painting order for
absolutely positioned boxes *inside* flex-items. Blink, according to
Blink bug 606208[1], disagrees with the spec here, so does WebKit.

We (as in Mozilla) rediscovered that issue by investigating real-world
breakage on an airline's date picker, where the disagreement caused
some issues. As far as I understand the spec, Firefox is right here,
so the WPT matches that, it's failing everywhere but Firefox.

According to the comment in the crbug, this may also be an issue for
Grid. And while this may look like a super weird edge case, I fear
there might be quite some cases where this difference results in some
subtle cross-browser issues.

Would be good to have some additional eyes on that. :)

[0]: https://wpt.fyi/results/css/css-flexbox/order/order-abs-children-painting-order.html
[1]: https://bugs.chromium.org/p/chromium/issues/detail?id=606208

Best,
Dennis

[Chris Harrelson]

+David Grogan WDYT?

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[David Grogan]

Now that we have a test (thanks Dennis!) we can add it to the list of compat2021 tests and it won't fall off our radar. Further, we'll prioritize it given that it has surfaced to mozilla on an airline site.

But if it's a painting issue that also affects grid, I won't realistically be able to fix it on my own and will have to rope in someone else.

[Chris Harrelson]

Oh I see - it's just a paint order bug at this point? +Philip Rogers for visibility about that.

[David Grogan]

On Thu, Jun 10, 2021 at 11:42 AM Chris Harrelson <chri...@google.com> wrote:

Oh I see - it's just a paint order bug at this point?

I'm not sure. I'm looking forward to pdr's assessment. Maybe layout needs to supply paint with something?

[Philip Rogers]

(resending after joining the group)

This is a paint bug where we don't consider the order property when iterating PaintLayers. I've updated https://crbug.com/606208 with details.