Whenever FocusState::mFocusLayersId changes, compute into another adjacent variable the LayersId of the child of the closest RefLayer ancestor of the focused layer if there is one. Provide a getter callable from elsewhere for obtaining that LayersId.

(Note: Maybe the way LayerIds work FocusState::mFocusLayersId is already this layer itself if FocusState::mFocusLayersId is defined to always be a child of a RefLayer. In that case, this bug is WORKSFORME.)

I think this is WFM. Ryan can you confirm?

I believe this is the way it currently works. A LayersId corresponds to a layer tree, so mFocusLayersId will correspond to the layer tree that contains the focused element.

Which if I understand correctly, is what you want

Closing as WFM per comment 2. Feel free to reopen if there's more that needs doing here.