--- a/client/webapp/js/engine/tile_grid_loader.js
+++ b/client/webapp/js/engine/tile_grid_loader.js
@@ -3,2 +3,7 @@
 // Exposes window.currentMapGrid, window.currentMapWidth, window.currentMapHeight, and window.getTileAt(tx, ty)
 
+const root = typeof window !== "undefined" ? window : (typeof globalThis !== "undefined" ? globalThis : global);
+if (typeof window === "undefined") {
+  root.window = root;
+}
+
@@ -40,2 +45,9 @@
 
+    const gridLength = width * height;
+    const minExpectedLength = 16 + poiCount * 3 + encounterCount * 5 + gridLength;
+    if (byteBuf.length < minExpectedLength) {
+      console.error("[TileGridLoader] Buffer truncated:", byteBuf.length, "needed:", minExpectedLength);
+      return null;
+    }
+
@@ -64,7 +76,2 @@
-    const gridLength = width * height;
-    if (offset + gridLength > byteBuf.length) {
-      console.error("[TileGridLoader] Buffer truncated for grid:", byteBuf.length, "needed:", offset + gridLength);
-      return null;
-    }
-
     const grid = byteBuf.subarray(offset, offset + gridLength);
-    window.currentMapGrid = grid;
-    window.currentMapWidth = width;
-    window.currentMapHeight = height;
-    window.currentMapMetadata = {
+    root.currentMapGrid = grid;
+    root.currentMapWidth = width;
+    root.currentMapHeight = height;
+    root.currentMapMetadata = {
@@ -88,4 +95,4 @@
     const grid = new Uint8Array(width * height);
     grid.fill(defaultTile);
-    window.currentMapGrid = grid;
-    window.currentMapWidth = width;
-    window.currentMapHeight = height;
-    window.currentMapMetadata = {
+    root.currentMapGrid = grid;
+    root.currentMapWidth = width;
+    root.currentMapHeight = height;
+    root.currentMapMetadata = {
@@ -106,4 +113,4 @@
-    const w = window.currentMapWidth || 0;
-    const h = window.currentMapHeight || 0;
-    const grid = window.currentMapGrid;
+    const w = root.currentMapWidth || 0;
+    const h = root.currentMapHeight || 0;
+    const grid = root.currentMapGrid;
     const ix = Math.floor(tx);
     const iy = Math.floor(ty);
-    if (!grid || ix < 0 || ix >= w || iy < 0 || iy >= h) {
+    if (!grid || !(ix >= 0 && ix < w && iy >= 0 && iy < h)) {
       return 2; // TileType.WALL
     }
@@ -118,3 +125,3 @@
-    const w = window.currentMapWidth || 0;
-    const h = window.currentMapHeight || 0;
-    const grid = window.currentMapGrid;
+    const w = root.currentMapWidth || 0;
+    const h = root.currentMapHeight || 0;
+    const grid = root.currentMapGrid;
@@ -131,3 +138,3 @@
-window.TileGridLoader = TileGridLoader;
-window.getTileAt = TileGridLoader.getTileAt;
-window.setTileAt = TileGridLoader.setTileAt;
+root.TileGridLoader = TileGridLoader;
+root.getTileAt = TileGridLoader.getTileAt;
+root.setTileAt = TileGridLoader.setTileAt;