Fix absolute-flow's auto positioning
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If an absolute positioned flow has no top, bottom, left, right property, its hypothetical box position should be the margin-end of its previous sibling, not the border-end.
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- [x] These changes fix#12676 (github issue number if applicable).
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Implement flexible box layout for row container
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This pull requests implements basic flexible box layout for row container.
It has implemented most basic flexbox features, including grow, shrink, multi-line, *reverse properties, and alignment under `justify-content`, `align-items`, `align-self`, `align-content`.
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r? @pcwalton
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Iterate over the items, create lines from them, resolve flex lengths
against them, set their inline size and margin lengths, and align them
according to 'justify-content', 'align-content' and 'align-self'.
Previously, the code that called this function was responsible for this.
Some code paths neglected to do it. This moves the damage removal to one
central location.
WebRender.
This happens asynchronously, just as it does in non-WebRender mode.
This functionality is a prerequisite for doing proper display-list-based
hit testing in WebRender, since it moves the scroll offsets into Servo
(and, specifically, into the script thread, enabling iframe event
forwarding) instead of keeping them private to WebRender.
Requires servo/webrender_traits#55 and servo/webrender#277.
Partially addresses #11108.
