After talking with Priscilla, I thought I might try something to
enhance the 'sliding tab' metaphor.
Perhaps we could show a little bit of the tab in the 'unchecked/
unoverlayed' state and then have the tab slide out on mouseover, more
clearly obscuring/covering the tag icon, as opposed to replacing it.
The slight protuberance might be enough to provoke users to try
mousing over it such that we can limit the mouseover hotspot to just
the collection icon area, rather than spreading it out across the
entire collection row.
Please put this animated gif in a browser to see what the interaction
might be like.

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