eg. at the end of a loop, the session may already be playing the
beginning of the loop. The TransportLooped signal was emitted.
Yet due to playback latency, the audible frame is still at the end of
the loop.
To interpolate the playhead position the UI needs to be able to know:
Relying on the TransportLooped signal is not sufficient because it
does not take playback latency into account.
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