If _engine.transport_frame() is not up to date, we emit Located in
Session::backend_sync_callback() because that's when audible_frame() is up to
date. We don't want to emit it twice, because then, the playhead jumps back and
forth.
* mouse click in the ruler -> jump to requested location
* mouse release -> jump to old location (because audible_frame has to catch up)
* backend_sync_callback() called -> jump to new location
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