Each tilemap layer has its own Z: an offset added to the Z of the placed tilemap, like a child's Z to its parent, default 0 keeps the previous order (layers right after their map, in list order), a negative Z sorts a layer below the map, on a tie layers keep their list order and follow the hierarchy against other objects; editor scene and game use the same order through the new pure function drawSlots, a tilemap draws into one container per layer, and the tilemap panel gains a Z field for the selected layer with one undo step per change (#174)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/e2e/40-tilemap.mjs

@@ -452,6 +452,39 @@ export async function tilemap(t) {
     JSON.stringify({ layers, renamedRow, nameNow }),
   );
 
+  // 40p. The Z field below the name gives the painted layer a z of its own, added to the map's
+  //      (#174): at −5 the upper layer is sorted in below the map itself and below the lower layer.
+  const zField = await inPanel(t, '__gm.tilemap.layerZAt()');
+  if (zField) {
+    await click(zField.x, zField.y);
+    await press('a', { ctrl: true });
+    await typeText('-5');
+    await press('Enter');
+  }
+  await settle();
+  await shot('40-tilemap-layer-z');
+  const withZ = await json(`__gm.tilemap.layers(${JSON.stringify(made.id)})`);
+  const slot = (order, id) => order.indexOf(id);
+  const orderZ = await json('__gm.drawOrder()');
+  const upperSlot = `${MAP_OBJECT}#${withZ[1].id}`;
+  const lowerSlot = `${MAP_OBJECT}#${withZ[0].id}`;
+  await undo();
+  await settle();
+  const orderBack = await json('__gm.drawOrder()');
+  const zBack = (await json(`__gm.tilemap.layers(${JSON.stringify(made.id)})`))[1].z;
+  check(
+    'the Z field sorts the painted layer below the map and the other layer, and undo puts it back',
+    withZ[1].z === -5 &&
+      withZ[0].z === 0 &&
+      slot(orderZ, upperSlot) >= 0 &&
+      slot(orderZ, upperSlot) < slot(orderZ, MAP_OBJECT) &&
+      slot(orderZ, upperSlot) < slot(orderZ, lowerSlot) &&
+      zBack === 0 &&
+      slot(orderBack, MAP_OBJECT) < slot(orderBack, lowerSlot) &&
+      slot(orderBack, lowerSlot) < slot(orderBack, upperSlot),
+    JSON.stringify({ withZ, orderZ, orderBack, zBack }),
+  );
+
   // Pixels again, so the animation is cut down to one step for the reading, as in 40e.
   await mutate("__gm.state.project.assets.find((a) => a.id === 'at-grass').pictures = ['at-floor-1']");
   await settle();