diff --git a/src/hooks/useDwgViewer.ts b/src/hooks/useDwgViewer.ts
index cad2d86..d4bf336 100644
--- a/src/hooks/useDwgViewer.ts
+++ b/src/hooks/useDwgViewer.ts
@@ -78,8 +78,14 @@ export function useDwgViewer() {
 
   // mount Viewer2D once
   useEffect(() => {
-    if (!containerRef.current) return;
-    const viewer = new Viewer2D(containerRef.current);
+    const container = containerRef.current;
+    if (!container) return;
+    // Viewer2D has no dispose/teardown API (permanent window/canvas
+    // listeners, its own rAF loop) — defensively clear any leftover canvas
+    // from a prior construction (e.g. a dev Fast Refresh remount) so it
+    // never stacks two live WebGL contexts in the same container.
+    container.replaceChildren();
+    const viewer = new Viewer2D(container);
     viewerRef.current = viewer;
     viewer.setTheme(true);
     viewer.onSelect((entity) => setSelectedEntity(entity));
diff --git a/src/main.tsx b/src/main.tsx
index dfacde0..2bf1b15 100644
--- a/src/main.tsx
+++ b/src/main.tsx
@@ -1,10 +1,10 @@
-import { StrictMode } from 'react';
 import { createRoot } from 'react-dom/client';
 import App from './App';
 import './index.css';
 
-createRoot(document.getElementById('root')!).render(
-  <StrictMode>
-    <App />
-  </StrictMode>,
-);
+// No <StrictMode>: Viewer2D (ported unchanged from the vanilla app) has no
+// dispose/teardown API and attaches permanent window/canvas listeners in its
+// constructor. StrictMode's dev-only mount→unmount→remount double-invoke
+// would construct it twice into the same container with no way to tear the
+// first instance down, leaving two overlapping WebGL canvases.
+createRoot(document.getElementById('root')!).render(<App />);