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[css-position-4] Add "afterRemoval" steps to top layer removal algorithms #12850
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            [css-position-4] Add "afterRemoval" steps to top layer removal algorithms #12850
                    keithamus
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| This should get approval from someone else before it lands 😄 | 
| Non-editorial LGTM. This should wait till the HTML PR is ready and get merged together probably? Not sure who is the spec editor for this CSS spec but they should check this PR too. | 
| I have requested review from both spec editors. | 
  
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Refs whatwg/html#11694.
The spec for implicit anchors is slightly wrong, as the timing of when an implicit anchor can be removed is awkward.
Implicit anchors for elements in the top layer need to be removed after transitions, otherwise the anchor positioning may cause reflow.
Chrome "cheats" and skips reflow if the implicit anchor is being removed, but this can cause glitches if the anchored element reflows during this transition, as it will appear to move around the page (losing its implicit anchoring) when reflow is triggered.
Rather than doing this, I propose we add a "list of steps |afterRemoval|" to the "request an element to be removed from the top layer (immediately)" steps. This way we can use this as a callback to determine when the implicit anchor association should be removed.