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>

src/core/model/Asset.ts

@@ -213,12 +213,15 @@ export interface TileAsset extends AssetBase {
 /**
  * One drawing layer of a tilemap: `cells` maps `"x,y"` to the id of the tile lying there, and a
  * layer nobody switched off is drawn. Which of that tile's quarters a cell shows is worked out
- * from its neighbours while drawing, never stored – see `model/tilemap.ts`.
+ * from its neighbours while drawing, never stored – see `model/tilemap.ts`. `z` is added to the `z`
+ * of the placed tilemap (#174), as a child's is to its parent's, so every placement of the map
+ * keeps its layers apart by the same steps; layers of equal `z` keep their list order.
  */
 export interface TilemapLayer {
   id: string;
   name: string;
   hidden?: boolean;
+  z?: number;
   cells: Record<string, string>;
 }
 

src/core/model/tilemap.ts

@@ -72,6 +72,11 @@ export function isLayerVisible(layer: TilemapLayer): boolean {
   return layer.hidden !== true;
 }
 
+/** How far a layer is drawn above its tilemap's own `z` (#174); a layer never given one sits on it. */
+export function layerZ(layer: TilemapLayer): number {
+  return layer.z ?? 0;
+}
+
 export function findLayer(asset: { layers: TilemapLayer[] }, layerId: string): TilemapLayer | undefined {
   return asset.layers.find((layer) => layer.id === layerId);
 }