Fix drag-drop crash + cover it with offscreen GUI tests

_drop_piece looked up plan.item(item.pid), but item.pid is a Placement id (pl2)
while CutPlan.item() expects a CutItem id (ci2) — every drop raised StopIteration
before validate/revert could run. Use the already-found placement's item_id
(plan.item(p.item_id)) for the stock-compat check and message.

Added tests/test_bom_window.py (offscreen QGraphicsScene): drop-overlap reverts
without crashing, drop-onto-incompatible-stock reverts, and a valid move commits.

128 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/woodshop/gui/bom_window.py

@@ -343,9 +343,10 @@ class BomWindow(QDialog):
         if target is None:
             target = sp_cur
         # Stock-type compatibility: a 2x4 can't go on a plywood sheet, etc.
-        if plan.item(item.pid).stock != target.stock:
+        item_stock = plan.item(p.item_id).stock
+        if item_stock != target.stock:
             self._revert(plan, item.pid, home)
-            self._status.setText(f"✗ {plan.item(item.pid).stock} can't go on {target.stock} — reverted")
+            self._status.setText(f"✗ {item_stock} can't go on {target.stock} — reverted")
             recompute(plan); self._refresh_all()
             return
         row_y0 = self._row_of(target.id)

tests/test_bom_window.py

@@ -0,0 +1,59 @@
+"""Offscreen tests for the BOM window's drag/drop path (no display needed —
+QGraphicsScene is pure Qt). Guards the placement-id vs cut-item-id crash."""
+import os
+
+import pytest
+
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+pytest.importorskip("PySide6")
+
+from PySide6.QtWidgets import QApplication  # noqa: E402
+
+from woodshop.cutplan import find_placement  # noqa: E402
+from woodshop.gui.bom_window import BomWindow, _Piece  # noqa: E402
+from woodshop.gui.controller import Controller  # noqa: E402
+
+_app = QApplication.instance() or QApplication([])
+
+
+def _pieces(w):
+    return sorted((it for it in w.scene.items() if isinstance(it, _Piece)),
+                  key=lambda it: it.pos().x())
+
+
+def test_drop_overlap_reverts_without_crashing(tmp_path):
+    c = Controller(str(tmp_path / "s.json"))
+    c.place("2x4", 30)
+    c.place("2x4", 30)                       # one stick, two pieces
+    w = BomWindow(c)
+    first, second = _pieces(w)[:2]
+    home = (second.sp_id, second.pos().x(), second.pos().y())
+    second.setPos(0, second.pos().y())       # drop on top of the first -> overlap
+    w._drop_piece(second, home)              # must not raise (was StopIteration)
+    assert "revert" in w._status.text().lower()
+
+
+def test_drop_onto_incompatible_stock_reverts(tmp_path):
+    c = Controller(str(tmp_path / "s.json"))
+    c.place("2x4", 24)
+    c.place("ply-3/4", 24, width_in=24)
+    w = BomWindow(c)
+    lumber = next(it for it in _pieces(w)
+                  if not find_placement(w._plan, it.pid)[0].is_sheet)
+    sheet_y = next(y0 for y0, _y1, sp in w._rows if sp.is_sheet)
+    home = (lumber.sp_id, lumber.pos().x(), lumber.pos().y())
+    lumber.setPos(10, sheet_y + 5)           # drag the 2x4 onto the plywood sheet
+    w._drop_piece(lumber, home)              # must not raise
+    assert "can't go" in w._status.text()
+
+
+def test_valid_move_commits(tmp_path):
+    c = Controller(str(tmp_path / "s.json"))
+    c.place("2x4", 20)
+    c.place("2x4", 20)
+    w = BomWindow(c)
+    second = _pieces(w)[1]
+    home = (second.sp_id, second.pos().x(), second.pos().y())
+    second.setPos(50 * w._px, second.pos().y())   # slide it right, still clear
+    w._drop_piece(second, home)
+    assert "placed" in w._status.text().lower()