Currently, Sync's FakeServer (used in many tests) runs on the UI thread. That's often convenient (easy to set things up in test code), but it can cause deadlocks between Sync thread and UI thread.
One such deadlock is being worked around (not actually fixed) by https://crrev.com/c/1156587, but probably other cases exist.

We should consider moving the FakeServer to its own thread to properly fix this issue.