Some additional context for Perry since I was the one who suggested cc-ing you: I don't know how much of the summary from Biran is specific to the current implementation vs parent-intercept (Bug 1231213). We discussed with Honza that we should probably focus on a solution that works for the new implementation, so I think you can assume parent-intercept = true for this discussion.
Bug 1563607 Comment 2 Edit History
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Some additional context for Perry since I was the one who suggested cc-ing you: I don't know how much of the summary from Brian is specific to the current implementation vs parent-intercept (Bug 1231213). We discussed with Honza that we should probably focus on a solution that works for the new implementation, so I think you can assume parent-intercept = true for this discussion.