The groove's facets came in sweep order, and the BSP takes each list's first polygon as its splitter, so the tree was as deep as the list: quadratic time and memory. An M8 x 60 mm thread ran out of tree depth after 12.9 GB. A fixed shuffle gives a shallow tree and the same result (volume and faces equal to 1e-9). Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>