Keep snap helper safe without context (#1828)

static/js/editor/snap.js

@@ -23,8 +23,10 @@
  * @returns {{x: number, y: number, guides: Array}}
  */
 export function computeSnap(layer, nx, ny, ctx) {
-  const SNAP_PX = 6 / Math.max(ctx.zoom, 0.0001);
-  const cw = ctx.canvasW, ch = ctx.canvasH;
+  if (!layer || !layer.canvas || !ctx) return { x: nx, y: ny, guides: [] };
+  const zoom = Number.isFinite(Number(ctx.zoom)) ? Number(ctx.zoom) : 1;
+  const SNAP_PX = 6 / Math.max(zoom, 0.0001);
+  const cw = Number(ctx.canvasW) || 0, ch = Number(ctx.canvasH) || 0;
   const w = layer.canvas.width, h = layer.canvas.height;
 
   const vTargets = [

tests/test_snap_other_layers_nonarray_js.py

@@ -36,6 +36,28 @@ def test_compute_snap_tolerates_non_array_other_layers():
     assert r["x"] == 10 and r["y"] == 10 and r["guides"] == []
 
 
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_compute_snap_tolerates_missing_layer_or_context():
+    js = f"""
+    import {{ computeSnap }} from '{_HELPER.as_posix()}';
+    console.log(JSON.stringify([
+      computeSnap(null, 10, 20, {{ zoom: 1, canvasW: 800, canvasH: 600 }}),
+      computeSnap({{ id: 'L1' }}, 11, 21, {{ zoom: 1, canvasW: 800, canvasH: 600 }}),
+      computeSnap({{ id: 'L1', canvas: {{ width: 100, height: 50 }} }}, 12, 22, null)
+    ]));
+    """
+    proc = subprocess.run(
+        ["node", "--input-type=module"],
+        input=js, capture_output=True, text=True, cwd=str(_REPO), timeout=30,
+    )
+    assert proc.returncode == 0, proc.stderr
+    assert json.loads(proc.stdout.strip()) == [
+        {"x": 10, "y": 20, "guides": []},
+        {"x": 11, "y": 21, "guides": []},
+        {"x": 12, "y": 22, "guides": []},
+    ]
+
+
 @pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
 def test_compute_snap_still_snaps_to_a_layer_edge():
     other = [{"id": "L2", "visible": True, "offset": {"x": 12, "y": 300},