Prevent missing images from black-screening canvas games

app.py

@@ -1236,6 +1236,24 @@ def embed_assets(html_code: str, assets: dict[str, str], specs: list[AssetSpec])
     HTMLImageElement.prototype.__generatedAssetMapper = true;
   }}
 
+  var originalDrawImage = CanvasRenderingContext2D.prototype.drawImage;
+  if (!CanvasRenderingContext2D.prototype.__generatedAssetDrawGuard) {{
+    CanvasRenderingContext2D.prototype.drawImage = function (image) {{
+      try {{
+        if (image instanceof HTMLImageElement) {{
+          var current = image.getAttribute("src") || image.src || "";
+          var mapped = pickAsset(current);
+          if (mapped && mapped !== current) image.src = mapped;
+          if (!image.complete || image.naturalWidth === 0 || image.naturalHeight === 0) return;
+        }}
+        return originalDrawImage.apply(this, arguments);
+      }} catch (error) {{
+        return;
+      }}
+    }};
+    CanvasRenderingContext2D.prototype.__generatedAssetDrawGuard = true;
+  }}
+
   window.addEventListener("DOMContentLoaded", function () {{
     document.querySelectorAll("img").forEach(function (img) {{
       var mapped = pickAsset(img.getAttribute("src") || img.src);