Bug 218579 – [Interop 2021][css-flexbox] flex item margins not included in flex container overflow

• Status: NEW
• Component: CSS

[fantasai]

Spec requires flex item margins (and the flex container's padding) to be included in the flex container's overflow area.
This is tested in 
  https://wpt.fyi/results/css/css-flexbox/overflow-area-001.html?label=experimental&label=master&aligned
  https://wpt.fyi/results/css/css-flexbox/overflow-area-002.html?label=experimental&label=master&aligned
and it definitely bugs authors. (The spec bugs about including margins/padding were some of the most flagged issues in the CSSWG repo.)

[Radar WebKit Bug Importer]

<rdar://problem/71046552>

[Tim Nguyen (:ntim)]

    if (isGridItem()) {
        // As per https://github.com/w3c/csswg-drafts/issues/3653, child's margins should contribute to the scrollable overflow area.
        // FIXME: Expand it to non-grid cases when applicable.
        rect.setWidth(rect.width() + std::max(0_lu, marginEnd()));
    }

in RenderBox::layoutOverflowRectForPropagation is a good first place to look at.