I'm not sure I follow the discussion in here:

https://codereview.chromium.org/1236913004

but we should definitely not have a Map for scroll customization callbacks. I don't know what the comment about not affording the size increase is about, we've added stuff to RareData lots since then, and we add RareData for all kinds of reasons. If ElementRareData is too big we can do things like move the PseudoElements into a sub object, or other space saving things. We shouldn't keep separate tracking maps.