Views UI Preview pager is broken

I bet it's from all the form controller code from November, so I'll at least write a test as my penance.

Steps to reproduce:

1. Create 2 nodes
2. Create a view of nodes, limit to 1, use a pager
3. Preview the view in the Views UI
4. Click the "next" link

Expected result:

The second node

Actual result:

A second Views UI Preview form, embedded in the Views UI Preview form.