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The other thing is: there are currently RUM providers which provide metrics based on longtasks. Given that, do we need to ship the new API with the solution to this...

Assigning to Steve, should be a one-line fix.

@acomminos this is blocking Mozilla's implementation, could you take a look?

I definitely think this is an important problem worth solving, and in particular for CLS we really do want any shifts occurring in iframes to become visible via analytics providers...

I think we could add an example with a diagram showing ~half the viewport shifting and explaining what the score would be in that case.

Feel free to ping when this is ready for review again

Note that you cannot expose the elements if those are inside shadow DOM. What's the plan in that case?

Exposing the closest ancestor that's not in shadow DOM makes sense (the shadow host can also itself be within a shadow tree, i.e. they can be nested). I don't know...

Yea that sounds like a bug to me. I think we should ignore content that was moved from being completely offscreen to partially or fully onscreen. Similarly, we should ignore...

Ah yes we need to penalize full viewport moves, though from a user perspective I still think that content that was entirely offscreen and now appearing onscreen is the same...