From https://drafts.csswg.org/css-overflow/#overflow-propagation:
> UAs must apply the overflow-* values set on the root element to the viewport.
> However, when the root element is an [HTML] html element (including XML syntax
> for HTML) whose overflow value is visible (in both axes), and that element has
> a body element as a child, user agents must instead apply the overflow-*
> values of the first such child element to the viewport. The element from which
> the value is propagated must then have a used overflow value of visible.
This was out of sync with Document::IsScrollingElement, which implements the
right thing.
Differential Revision: https://phabricator.services.mozilla.com/D49196
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extra : moz-landing-system : lando
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