During drag operations, validateNewPlacements() compared each solver
result against the pre-drag positions saved once in preDrag(). As the
user dragged further, the cumulative rotation from that fixed baseline
easily exceeded the 91-degree threshold, causing valid intermediate
results to be rejected with 'flipped orientation' warnings and making
parts appear to explode.
Fix: call savePlacementsForUndo() after each accepted drag step so
that the flip check compares against the last accepted state rather
than the original pre-drag origin.
