Re: [w3ctag/design-reviews] Review request for ResizeObserver (#187)

> currently only Edge layout engine uses fragments internally. Without engine support, observing fragments cannot be done efficiently.

Gecko also has a notion of "prev/next in flow" which allows traversal of all fragmented boxes very efficiently.

> most of the web content does not involve fragments, and observing traditional css rect works well enough for this case.

This is changing, given multicol layout, and will continue to change as paged overflow and regions get implemented. Expect custom layout to get fragmentation support as well (if not in V1) leading to all sorts of new ways to fragment content.

I'm very reluctant to have new APIs continue the incorrect assumption made in the older DOM APIs of presuming a single box for each node. This will be a disservice in the long run and will work to prevent adoption (and implementation) of newer page layout techniques. If nothing else, craft the API to handle multiple boxes per node and allow current implementations to only return the first one, or the bounding rect (but then be clear it's a bounding rect), so that multiple boxes can be seamlessly handled in the future.

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