The Move dialog's predecessor check flagged every unfinished step before
the destination — including the from_step itself, which is in-progress by
definition when advancing partial parts out of it. So any "Send → next"
to a not-yet-started step showed a hard "Predecessor not done: <from_step>"
blocker and greyed out SEND (reproduced on WO-30061: Racking → Ready for
processing). This broke partial advance for ALL quantities, not just
1-part orders.
Fix: _blockers_for_move only blocks unfinished steps STRICTLY BETWEEN
from_step and to_step (you'd be skipping an incomplete intermediate
stage). Immediate-next advance is allowed; skip-ahead still blocked;
backward (rework) moves unblocked. Verified on entech: blocker no longer
fires for Racking → Ready for processing.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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