If disk-monitoring is disabled: disk-reader position is not relevant.
If Rec-arm is off: disk-writer position is not relevant.
But...
Play -> [plugins] -> Record
is basically a bounce and best done using the bounce operation.
(faster than realtime).
Input + Play -> Record -> Output
Ardour would need to align playback with the Input to be recorded
and at the same time align it with output, so that a player can play
along on the same track. That's not possible without a time-machine (or
a 2nd play processor).
While it can work in theory under some special circumstances, allowing
the disk-reader before the disk-writer is really just confusing,
error prone and valid uses cases are better handled by dedicated
operations.
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