Starting work on the following tests.

* The first failure was caused by not properly dropping the references,
which is now fixed.
* The next failure is caused by a `cx.after_window_update` being changed
to `cx.on_next_frame`. This doesn't seem to work (at least in tests),
but might also explain the flaky following behavior Max and I observed
if there's a timing bug in production too.
* * When you are following someone and they move their cursor you should
receive two proto messages: UpdateBuffer to move the cursor, and
UpdateFollowers to update where you're following. It seems like this
could be made a bit less chatty/fragile, but probably out of scope for
this.
* With that worked around, there's one more failure, but I haven't
looked into it yet. (possibly caused by an incorrect fix for the cx
one).