deadlock in movingranges branch after I tried to execute a signature assistant

[David Nolden]

Hmm, looks like we need to release the foreground-lock in
waitForUpdate while blocking (if it's locked). I don't have time right
now, will take a look later. In case that you adapt it, it should also
be stated in the comment of waitForUpdate that the foreground lock is
temporarily released during the waiting. It means that we have to be
careful when we call waitForUpdate, as the before/after states may
differ. I don't see another way though ATM.