Rerender ContentBranchNodes when the selection anchor leaves

This causes unicorns to disappear, hopefully allowing us to
get rid of some of the bookkeeping previously used to make that
happen.

It will also clean up redundant chimeras, once we start using those.

Change-Id: I95632cdab95f430540e2ed682a706d6cf371077f
Merge "Rerender ContentBranchNodes when the selection anchor leaves"