Before this change, we were not detaching paintables from DOM nodes
within shadow subtrees.
This appears to be the main reason that keyboard editing was doing
immediate forced relayout: doing a full layout invalidation meant we'd
build a new layout tree, which then hid the problem with with
still-attached paintables.
By detaching them before committing a new layout, we make it possible
for keyboard editing to just use normal relayout, instead of full forced
invalidation & relayout.
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