diff --git a/packages/studio/src/components/nle/useCompositionStack.test.tsx b/packages/studio/src/components/nle/useCompositionStack.test.tsx
index a3d300bc79..d40a5abaa8 100644
--- a/packages/studio/src/components/nle/useCompositionStack.test.tsx
+++ b/packages/studio/src/components/nle/useCompositionStack.test.tsx
@@ -42,3 +42,72 @@ describe("useCompositionStack — project scoping", () => {
     });
   }
 });
+
+describe("useCompositionStack — activating a composition by path", () => {
+  afterEach(() => {
+    document.body.innerHTML = "";
+  });
+
+  function mountStack(activeCompositionPath: string | null) {
+    const host = document.createElement("div");
+    document.body.append(host);
+    const root = createRoot(host);
+    const seen: { stack: ReturnType<typeof useCompositionStack>["compositionStack"] } = {
+      stack: [],
+    };
+
+    function Harness(props: { activeCompositionPath: string | null }) {
+      seen.stack = useCompositionStack({ projectId: "p", ...props }).compositionStack;
+      return null;
+    }
+
+    return { root, seen, Harness, activeCompositionPath };
+  }
+
+  // The root stays on the master level; everything else pushes a second level.
+  for (const path of [
+    "compositions/scene-a.html",
+    "parts/part-1.html",
+    "chapter-2.html",
+    "a/b/c/deep.html",
+  ]) {
+    it(`pushes a level for ${path}`, async () => {
+      const { root, seen, Harness } = mountStack(path);
+
+      await act(async () => {
+        root.render(<Harness activeCompositionPath={path} />);
+      });
+
+      expect(seen.stack).toHaveLength(2);
+      expect(seen.stack[1]?.id).toBe(path);
+      expect(seen.stack[1]?.previewUrl).toBe(`/api/projects/p/preview/comp/${path}`);
+
+      act(() => root.unmount());
+    });
+  }
+
+  it("labels a non-compositions/ path without mangling it", async () => {
+    const { root, seen, Harness } = mountStack("parts/part-1.html");
+
+    await act(async () => {
+      root.render(<Harness activeCompositionPath="parts/part-1.html" />);
+    });
+
+    expect(seen.stack[1]?.label).toBe("parts/part-1");
+
+    act(() => root.unmount());
+  });
+
+  it("keeps the master alone for the root composition", async () => {
+    const { root, seen, Harness } = mountStack("index.html");
+
+    await act(async () => {
+      root.render(<Harness activeCompositionPath="index.html" />);
+    });
+
+    expect(seen.stack).toHaveLength(1);
+    expect(seen.stack[0]?.id).toBe("master");
+
+    act(() => root.unmount());
+  });
+});
diff --git a/packages/studio/src/components/nle/useCompositionStack.ts b/packages/studio/src/components/nle/useCompositionStack.ts
index e210184637..feb947c42d 100644
--- a/packages/studio/src/components/nle/useCompositionStack.ts
+++ b/packages/studio/src/components/nle/useCompositionStack.ts
@@ -108,7 +108,13 @@ export function useCompositionStack({
     if (activeCompositionPath === "index.html") {
       usePlayerStore.getState().setElements([]);
       updateCompositionStack([master]);
-    } else if (activeCompositionPath && activeCompositionPath.startsWith("compositions/")) {
+    } else if (activeCompositionPath) {
+      // Any composition file that isn't the root, wherever it lives. Gating
+      // this on a `compositions/` prefix meant a project laying its comps out
+      // anywhere else (`parts/part-1.html`, generated multi-part builds) hit
+      // no branch at all: the stack kept the master mounted while the Comps
+      // panel highlighted the row, so the canvas and timeline stayed on
+      // index.html and edits landed in the root file.
       const label = activeCompositionPath.replace(/^compositions\//, "").replace(/\.html$/, "");
       const previewUrl = `/api/projects/${projectId}/preview/comp/${encodePreviewPath(activeCompositionPath)}`;
       usePlayerStore.getState().setElements([]);
@@ -116,7 +122,7 @@ export function useCompositionStack({
         if (prev[prev.length - 1]?.id === activeCompositionPath) return prev;
         return [master, { id: activeCompositionPath, label, previewUrl }];
       });
-    } else if (!activeCompositionPath) {
+    } else {
       usePlayerStore.getState().setElements([]);
       updateCompositionStack([master]);
     }