diff --git a/frontend/src/lib/history.subview.dom.test.ts b/frontend/src/lib/history.subview.dom.test.ts
index c7d96658..3ce4e66e 100644
--- a/frontend/src/lib/history.subview.dom.test.ts
+++ b/frontend/src/lib/history.subview.dom.test.ts
@@ -63,7 +63,7 @@ describe("useHistorySubview seed", () => {
   });
 
   it("ignores the URL entirely when no SubviewUrl is supplied (state-only mirror)", () => {
-    setUrl("?v=a", { _sub: "fromState" });
+    setUrl("?v=a", { _sub: "fromState", _subOf: "" });
     const { result } = renderHook(() => useHistorySubview("root"));
     expect(result.current.view).toBe("fromState");
     // A URL param with no url config must NOT leak in.
@@ -128,6 +128,55 @@ describe("useHistorySubview open", () => {
   });
 });
 
+// One entry, two modules: the sign-in card mounts on the entry a Plex season stood on (signed out
+// there), and Plex on one the card's register view stood on (signed in from it). Each reads only its
+// own sub-view: another's is not one of its views (the card rendered a mode it has no button for).
+describe("useHistorySubview on an entry another module's sub-view stood on", () => {
+  const other: SubviewUrl<string> = {
+    key: "m",
+    encode: (v) => (v === "signin" ? null : v),
+    decode: (raw) => (raw === "register" ? raw : null),
+  };
+
+  it("opens at its own root, not on the other's view", () => {
+    const plex = renderHook(() => useHistorySubview("root", url));
+    act(() => plex.result.current.open("season"));
+    plex.unmount();
+    const card = renderHook(() => useHistorySubview("signin", other));
+    expect(card.result.current.view).toBe("signin");
+  });
+
+  it("the other way round too", () => {
+    const card = renderHook(() => useHistorySubview("signin", other));
+    act(() => card.result.current.open("register"));
+    card.unmount();
+    window.history.replaceState(window.history.state, "", "/");
+    const plex = renderHook(() => useHistorySubview("root", url));
+    expect(plex.result.current.view).toBe("root");
+  });
+
+  it("an older build's entry, which says nothing: its own only when its address names its view", () => {
+    setUrl("?v=season", { _sub: "season" });
+    expect(renderHook(() => useHistorySubview("signin", other)).result.current.view).toBe("signin");
+    expect(renderHook(() => useHistorySubview("root", url)).result.current.view).toBe("season");
+  });
+
+  it("reads its own from an entry it rewrote in place, which no address names", () => {
+    const first = renderHook(() => useHistorySubview("root"));
+    act(() => first.result.current.open("x", { replace: true }));
+    first.unmount();
+    expect(renderHook(() => useHistorySubview("root")).result.current.view).toBe("x");
+  });
+
+  it("still reads its own", () => {
+    const plex = renderHook(() => useHistorySubview("root", url));
+    act(() => plex.result.current.open("season"));
+    plex.unmount();
+    window.history.replaceState(window.history.state, "", "/");
+    expect(renderHook(() => useHistorySubview("root", url)).result.current.view).toBe("season");
+  });
+});
+
 describe("useHistorySubview popstate", () => {
   it("restores the sub-view from state._sub on Back/Forward", () => {
     const { result } = renderHook(() => useHistorySubview("root", url));
diff --git a/frontend/src/lib/history.ts b/frontend/src/lib/history.ts
index 8086f7b7..182cb61c 100644
--- a/frontend/src/lib/history.ts
+++ b/frontend/src/lib/history.ts
@@ -139,6 +139,9 @@ export interface SubviewUrl<T> {
   entry?: (view: T) => T;
 }
 
+/** Whose sub-view an entry's `_sub` is: the key its module mirrors to (a state-only one has none). */
+const subOwner = (url: { key: string } | undefined): string => url?.key ?? "";
+
 export function useHistorySubview<T>(
   root: T,
   url?: SubviewUrl<T>,
@@ -158,14 +161,18 @@ export function useHistorySubview<T>(
   // ⚠️ The seed AND the popstate handler must use THIS same fallback: a popstate can land on a
   // bookmark entry (URL-only, no _sub) — e.g. after an overlay's programmatic history.back() — and
   // reading history.state alone would wrongly collapse the bookmarked sub-view to root.
+  // The entry's _sub is this module's only when it says so (`_subOf`): one entry can carry another
+  // module's — the sign-in card mounts on the entry a Plex season stood on when signed out there, and
+  // took that season for one of its modes (a card with no button for it). An entry an older build
+  // wrote says nothing: its _sub is this module's when its address names one of this module's views.
   const readView = (): T => {
-    const fromState = window.history.state?._sub as T | undefined;
+    const st = window.history.state;
+    const raw = url ? new URLSearchParams(window.location.search).get(url.key) : null;
+    const decoded = url && raw != null ? url.decode(raw) : null;
+    const owned = st?._subOf === undefined ? decoded != null : st._subOf === subOwner(url);
+    const fromState = owned ? (st?._sub as T | undefined) : undefined;
     if (fromState != null) return fromState;
-    if (url) {
-      const raw = new URLSearchParams(window.location.search).get(url.key);
-      const decoded = raw != null ? url.decode(raw) : null;
-      if (decoded != null) return decoded;
-    }
+    if (decoded != null) return decoded;
     return root;
   };
   const [view, setView] = useState<T>(readView);
@@ -211,7 +218,7 @@ export function useHistorySubview<T>(
       // A fresh sfScroll so the opened sub-view is its own scroll entry (starts at the top, and Back
       // restores the view we came from) — then notify PageScroller, which this push doesn't re-render.
       window.history.pushState(
-        { ...rememberNavPosition(), _sub: kept, sfScroll: nextNavId() },
+        { ...rememberNavPosition(), _sub: kept, _subOf: subOwner(url), sfScroll: nextNavId() },
         "",
         href,
       );
@@ -253,7 +260,7 @@ export function useHistorySubview<T>(
 function writeSubEntry<T>(next: T, url: SubviewUrl<T> | undefined): void {
   const kept = url?.entry ? url.entry(next) : next;
   const href = url ? urlWithParams({ [url.key]: url.encode(next) }) : "";
-  replaceView({ ...window.history.state, _sub: kept }, href);
+  replaceView({ ...window.history.state, _sub: kept, _subOf: subOwner(url) }, href);
 }
 
 // --- Overlays / modals (one history entry each, nesting-safe) -------------------------------
diff --git a/frontend/src/modules/plex/PlexBrowse.playShow.dom.test.tsx b/frontend/src/modules/plex/PlexBrowse.playShow.dom.test.tsx
index 18af5433..152c2412 100644
--- a/frontend/src/modules/plex/PlexBrowse.playShow.dom.test.tsx
+++ b/frontend/src/modules/plex/PlexBrowse.playShow.dom.test.tsx
@@ -45,7 +45,7 @@ beforeAll(() => {
   globalThis.ResizeObserver ??= Stub as unknown as typeof ResizeObserver;
   globalThis.IntersectionObserver ??= Stub as unknown as typeof IntersectionObserver;
 });
-beforeEach(() => window.history.replaceState({ _sub: { kind: "grid" } }, ""));
+beforeEach(() => window.history.replaceState({ _sub: { kind: "grid" }, _subOf: "v" }, ""));
 afterEach(() => {
   cleanup();
   vi.restoreAllMocks();