The old system effectively cached cross-block
boundary information forever and never expired
them, even if the node creating them was
removed (that would have been easy to fix) or
an obstacle placed in the path (that would have
been difficult to fix with the old method).
Instead just do the simplest thing and split this
into a separate loop, and run a 2-second DNT with
a separate forward-trace from all active optic
nodes.
Doing a second forward trace for each node output
may not be as efficient but ultimately we create more
problems than we solve when we try to be too clever.
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