diff --git a/web/workspace/src/lib/workspace/navigation/ObserveDeferredHarness.svelte b/web/workspace/src/lib/workspace/navigation/ObserveDeferredHarness.svelte
new file mode 100644
index 000000000..0ff8ec8af
--- /dev/null
+++ b/web/workspace/src/lib/workspace/navigation/ObserveDeferredHarness.svelte
@@ -0,0 +1,14 @@
+<script lang="ts">
+  import { observeDeferred } from './observe-deferred.svelte.ts';
+  import type { DeferredResult } from './deferred.ts';
+
+  let { identity, read }: {
+    identity: string | undefined;
+    read: () => Promise<DeferredResult<string>>;
+  } = $props();
+  const loaded = observeDeferred(() => read(), () => identity);
+</script>
+
+<p data-testid="failure">{loaded.failure?.message ?? 'no failure'}</p>
+<p data-testid="data">{loaded.data ?? 'no data'}</p>
+<p data-testid="loading">{String(loaded.loading)}</p>
diff --git a/web/workspace/src/lib/workspace/navigation/observe-deferred.browser.spec.ts b/web/workspace/src/lib/workspace/navigation/observe-deferred.browser.spec.ts
new file mode 100644
index 000000000..b42ff90b2
--- /dev/null
+++ b/web/workspace/src/lib/workspace/navigation/observe-deferred.browser.spec.ts
@@ -0,0 +1,91 @@
+// @vitest-environment happy-dom
+import { cleanup, render, waitFor } from "@testing-library/svelte";
+import { afterEach, expect, test, vi } from "vitest";
+import { tick } from "svelte";
+import ObserveDeferredHarness from "./ObserveDeferredHarness.svelte";
+import { type DeferredResult, settle } from "./deferred.ts";
+
+afterEach(cleanup);
+
+function gate() {
+  let resolve!: (value: DeferredResult<string>) => void;
+  const promise = new Promise<DeferredResult<string>>((r) => {
+    resolve = r;
+  });
+  return { promise, resolve };
+}
+
+test("missing route identity neither dereferences empty state nor reads removed layout data", () => {
+  const read = vi.fn(() => {
+    throw new Error("layout data is no longer available");
+  });
+  const view = render(ObserveDeferredHarness, { identity: undefined, read });
+  expect(view.getByTestId("failure").textContent).toBe("no failure");
+  expect(view.getByTestId("data").textContent).toBe("no data");
+  expect(read).not.toHaveBeenCalled();
+});
+
+test("leaving a loaded Workspace hides its data and can enter another Workspace", async () => {
+  const view = render(ObserveDeferredHarness, {
+    identity: "team",
+    read: () => settle("Team"),
+  });
+  await view.findByText("Team");
+  const removed = vi.fn(() => {
+    throw new Error("removed layout data");
+  });
+  await view.rerender({ identity: undefined, read: removed });
+  expect(view.getByTestId("failure").textContent).toBe("no failure");
+  expect(view.getByTestId("data").textContent).toBe("no data");
+  expect(removed).not.toHaveBeenCalled();
+  await view.rerender({
+    identity: "other",
+    read: () => settle("Other Workspace"),
+  });
+  await view.findByText("Other Workspace");
+});
+
+test("late results after leaving a Workspace cannot restore its data or failure", async () => {
+  for (
+    const result of [
+      { ok: true, data: "Old Workspace" },
+      {
+        ok: false,
+        failure: { message: "Old failure", status: 403, redirect: null },
+      },
+    ] satisfies DeferredResult<string>[]
+  ) {
+    const pending = gate();
+    const view = render(ObserveDeferredHarness, {
+      identity: "team",
+      read: () => pending.promise,
+    });
+    await view.rerender({ identity: undefined, read: () => pending.promise });
+    pending.resolve(result);
+    await tick();
+    expect(view.getByTestId("failure").textContent).toBe("no failure");
+    expect(view.getByTestId("data").textContent).toBe("no data");
+    view.unmount();
+  }
+});
+
+test("same-Workspace refresh retains data, while denied refresh and identity changes hide it", async () => {
+  const view = render(ObserveDeferredHarness, {
+    identity: "team",
+    read: () => settle("Team"),
+  });
+  await view.findByText("Team");
+  const refresh = gate();
+  await view.rerender({ identity: "team", read: () => refresh.promise });
+  expect(view.getByTestId("data").textContent).toBe("Team");
+  refresh.resolve({
+    ok: false,
+    failure: { message: "Denied", status: 403, redirect: null },
+  });
+  await view.findByText("Denied");
+  expect(view.getByTestId("data").textContent).toBe("no data");
+  await view.rerender({ identity: undefined, read: () => refresh.promise });
+  await waitFor(() =>
+    expect(view.getByTestId("failure").textContent).toBe("no failure")
+  );
+});
diff --git a/web/workspace/src/lib/workspace/navigation/observe-deferred.svelte.ts b/web/workspace/src/lib/workspace/navigation/observe-deferred.svelte.ts
index 82458c7aa..2339d8f44 100644
--- a/web/workspace/src/lib/workspace/navigation/observe-deferred.svelte.ts
+++ b/web/workspace/src/lib/workspace/navigation/observe-deferred.svelte.ts
@@ -3,7 +3,7 @@ import type { DeferredFailure, DeferredResult } from "./deferred.ts";
 // 同じ対象の再取得では確定済み表示を保持し、対象切替では旧値を即座に隠す。
 export function observeDeferred<T>(
   read: () => Promise<DeferredResult<T>>,
-  identity: () => string,
+  identity: () => string | undefined,
 ) {
   let settled = $state.raw<{ identity: string; value: T } | null>(null);
   let failure = $state.raw<{ identity: string; value: DeferredFailure } | null>(
@@ -12,6 +12,12 @@ export function observeDeferred<T>(
   let loading = $state(true);
   $effect(() => {
     const key = identity();
+    // Leaving a scoped route can remove its params before the layout is destroyed.
+    if (key === undefined) {
+      loading = false;
+      failure = null;
+      return;
+    }
     const promise = read();
     let active = true;
     loading = true;
@@ -31,10 +37,12 @@ export function observeDeferred<T>(
   });
   return {
     get data() {
-      return settled?.identity === identity() ? settled.value : null;
+      const key = identity();
+      return settled !== null && settled.identity === key ? settled.value : null;
     },
     get failure() {
-      return failure?.identity === identity() ? failure.value : null;
+      const key = identity();
+      return failure !== null && failure.identity === key ? failure.value : null;
     },
     get loading() {
       return loading;