fix(plex): the backdrop crossfade toggle wins over reduced motion once set

prefers-reduced-motion hard-disabled the rotation whatever the toggle said,
and Windows reports it whenever its animation effects are off — so the
owner never saw the crossfade. Untouched, the setting now follows the
system (and the toggle shows that state); a pick made on the toggle is
stored and wins on every device.

frontend/src/lib/plexBackdrop.dom.test.tsx

@@ -2,12 +2,17 @@
 //
 // E5 S8: the crossfading art backdrop hook. Covers the pieces that don't need real layout — that a
 // pool paints #root + isolates it (so the z-index:-1 overlay can paint above #root's own background),
-// creates exactly one overlay layer when it may animate, and HARD-OFFS the rotation (no overlay, first
+// creates exactly one overlay layer when it may animate, and stays static (no overlay, first image
-// image only) under prefers-reduced-motion, a single image, or when disabled. matchMedia is stubbed
+// only) for a single image, the crossfade off, or when disabled. Then the crossfade setting itself:
-// per-test; the hook reads it at effect time.
+// untouched it follows prefers-reduced-motion, and the reader's own pick wins over it (the owner, whose
+// Windows animation effects are off, never saw the crossfade — 2026-10-03). matchMedia is stubbed per test.
 import { describe, it, expect, afterEach, vi } from "vitest";
-import { render, cleanup } from "@testing-library/react";
+import { render, cleanup, renderHook, act } from "@testing-library/react";
-import { useCrossfadeBackdrop } from "./plexDetail";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { createElement, type ReactNode } from "react";
+import { api } from "./api";
+import { qk } from "./queryKeys";
+import { useCrossfadeBackdrop, useDetailPrefs } from "./plexDetail";
 
 function mockReducedMotion(reduce: boolean) {
   window.matchMedia = vi.fn().mockImplementation((query: string) => ({
@@ -64,12 +69,11 @@ describe("useCrossfadeBackdrop", () => {
     expect(overlays().length).toBe(1);
   });
 
-  it("hard-offs the rotation under prefers-reduced-motion (static, no overlay)", () => {
+  it("animates under prefers-reduced-motion once the reader's setting says so", () => {
     withRoot();
     mockReducedMotion(true);
     render(<Harness arts={["/a.jpg", "/b.jpg"]} />);
-    expect(root().style.backgroundImage).toContain('url("/a.jpg")'); // first image still painted
+    expect(overlays().length).toBe(1);
-    expect(overlays().length).toBe(0); // but no crossfade layer
   });
 
   it("stays static for a single image (no overlay)", () => {
@@ -101,3 +105,44 @@ describe("useCrossfadeBackdrop", () => {
     expect(root().style.isolation).toBe("");
   });
 });
+
+describe("the crossfade setting", () => {
+  const prefsWith = (preferences: Record<string, unknown>) => {
+    const qc = new QueryClient();
+    qc.setQueryData(qk.me(), { preferences });
+    return ({ children }: { children: ReactNode }) =>
+      createElement(QueryClientProvider, { client: qc }, children);
+  };
+
+  it.each([
+    [false, true],
+    [true, false],
+  ])("untouched, follows the system (reduced motion %s → crossfade %s)", (reduce, on) => {
+    mockReducedMotion(reduce);
+    const { result } = renderHook(() => useDetailPrefs(), { wrapper: prefsWith({}) });
+    expect(result.current.crossfade).toBe(on);
+  });
+
+  it("keeps the reader's own pick over the system", () => {
+    mockReducedMotion(true);
+    const on = renderHook(() => useDetailPrefs(), {
+      wrapper: prefsWith({ plexInfoArtCrossfade: true }),
+    });
+    expect(on.result.current.crossfade).toBe(true);
+    mockReducedMotion(false);
+    const off = renderHook(() => useDetailPrefs(), {
+      wrapper: prefsWith({ plexInfoArtCrossfade: false }),
+    });
+    expect(off.result.current.crossfade).toBe(false);
+  });
+
+  it("turns on from the state in force, and stores that as the reader's pick", () => {
+    mockReducedMotion(true);
+    const save = vi.spyOn(api, "savePrefs").mockResolvedValue({} as never);
+    const { result } = renderHook(() => useDetailPrefs(), { wrapper: prefsWith({}) });
+    act(() => result.current.toggleCrossfade());
+    expect(result.current.crossfade).toBe(true);
+    expect(save).toHaveBeenCalledWith({ plexInfoArtCrossfade: true });
+    save.mockRestore();
+  });
+});

frontend/src/lib/plexDetail.ts

@@ -1,4 +1,4 @@
-import { useLayoutEffect } from "react";
+import { useLayoutEffect, useSyncExternalStore } from "react";
 import { usePref, useSavePref } from "./usePref";
 import type { TFunction } from "i18next";
 import type { PlexCard, PlexTmdb } from "./api";
@@ -7,6 +7,18 @@ import type { PlexCard, PlexTmdb } from "./api";
 // prefs, the TMDB backdrop URL, the crossfading art backdrop hook and the episode label. Split out so
 // plexDetailUi exports only components (Fast Refresh).
 
+const REDUCED_MOTION = "(prefers-reduced-motion: reduce)";
+const reducedMotion = () =>
+  typeof window !== "undefined" &&
+  typeof window.matchMedia === "function" &&
+  window.matchMedia(REDUCED_MOTION).matches;
+const onReducedMotionChange = (cb: () => void) => {
+  if (typeof window === "undefined" || typeof window.matchMedia !== "function") return () => {};
+  const m = window.matchMedia(REDUCED_MOTION);
+  m.addEventListener("change", cb);
+  return () => m.removeEventListener("change", cb);
+};
+
 export function useDetailPrefs() {
   // Three per-user detail-page toggles, each a server pref adopted on login (usePref). Default ON:
   // undefined → true, only an explicit false hides. The detail pages mount after `me` is cached, so
@@ -14,9 +26,16 @@ export function useDetailPrefs() {
   const [artBg, setArtBg] = usePref<boolean>("plexInfoArtBg", true);
   const [showCast, setShowCast] = usePref<boolean>("plexInfoCast", true);
   const [showRelated, setShowRelated] = usePref<boolean>("plexInfoRelated", true);
-  // E5 S8: slowly cross-dissolve the art backdrop through the item's backdrops/related art. Independent
+  // E5 S8: slowly cross-dissolve the art backdrop through the item's backdrops/related art. Untouched,
-  // of prefers-reduced-motion, which hard-disables the rotation regardless of this toggle.
+  // it follows the system: off where the reader asked for less motion (prefers-reduced-motion — which
-  const [crossfade, setCrossfade] = usePref<boolean>("plexInfoArtCrossfade", true);
+  // Windows also reports whenever its animation effects are off). Once the reader turns it on or off
+  // here, that pick wins on every device: the owner never saw the crossfade, because the system signal
+  // used to switch it off whatever the toggle said (2026-10-03). The toggle shows the state in force.
+  const reduce = useSyncExternalStore(onReducedMotionChange, reducedMotion, () => false);
+  const [crossfadePick, setCrossfade] = usePref<boolean | null>("plexInfoArtCrossfade", null, {
+    parse: (raw) => (typeof raw === "boolean" ? raw : null),
+  });
+  const crossfade = crossfadePick ?? !reduce;
   const savePref = useSavePref();
   return {
     artBg,
@@ -103,8 +122,8 @@ export function tmdbBackdropUrl(path: string): string {
 // content, so the dissolve is visible under the chrome. Once faded, the settled image is committed to
 // #root and the overlay reset to transparent — a double buffer, so only ever ONE extra DOM node.
 //
-// The rotation is HARD-OFF under prefers-reduced-motion (shows only the first image, no movement) and
+// The rotation runs only when `animate` says so — the reader's crossfade setting, which already folds in
-// when there are fewer than two images or it's disabled; `attachment:fixed` keeps it viewport-anchored
+// prefers-reduced-motion (useDetailPrefs) — and there are at least two images; `attachment:fixed` keeps it viewport-anchored
 // while the content scrolls. Cleared on unmount / disable / empty pool.
 const BACKDROP_FADE_MS = 1600;
 const BACKDROP_INTERVAL_MS = 7000;
@@ -148,13 +167,9 @@ export function useCrossfadeBackdrop(
     }
     paint(pool[0]!);
 
-    const reduce =
+    // Static case (one image / crossfade off): still MUST clear on unmount — otherwise #root keeps the
-      typeof window !== "undefined" &&
+    // painted backdrop after leaving the page (e.g. switching to another module).
-      typeof window.matchMedia === "function" &&
+    if (!animate || pool.length < 2) return clear;
-      window.matchMedia("(prefers-reduced-motion: reduce)").matches;
-    // Static case (one image / crossfade off / reduced motion): still MUST clear on unmount — otherwise
-    // #root keeps the painted backdrop after leaving the page (e.g. switching to another module).
-    if (!animate || reduce || pool.length < 2) return clear;
 
     const layer = document.createElement("div");
     Object.assign(layer.style, {