diff --git a/apps/web/src/motion.e2e.test.ts b/apps/web/src/motion.e2e.test.ts
index ff37c10..7c0ed4a 100644
--- a/apps/web/src/motion.e2e.test.ts
+++ b/apps/web/src/motion.e2e.test.ts
@@ -249,4 +249,74 @@ run("navigation motion (headless Chrome)", () => {
     await chrome.send("Input.dispatchKeyEvent", { type: "keyUp", key: "Escape", code: "Escape", windowsVirtualKeyCode: 27 });
     expect(await exists(".lr-menu")).toBe(false);
   }, 40_000);
+
+  test("prefers-reduced-motion: no view transition, animation or transition anywhere, and everything still works", async () => {
+    await setReducedMotion(true);
+    const still = async (what: string) => {
+      await chrome.eval(afterFrame);
+      await Bun.sleep(250);
+      const r = await m();
+      expect([what, r.vt, r.anims, r.trans, await running()]).toEqual([what, 0, [], [], []]);
+      await reset();
+    };
+    try {
+      // library to course
+      await open("/", 'a[href="/c/sample"]');
+      await click('a[href="/c/sample"]');
+      await until(() => exists(".lr-main > article h1"), "course page");
+      await still("library to course");
+
+      // course to lesson, then Next: the old lesson is still held while the next loads, then swapped plainly
+      await click('.lr-les[href="/c/sample/m1-intro"]');
+      await until(async () => (await text(".lr-main .lr-lesson h1.title")) === "Introduction", "intro");
+      await still("course to lesson");
+      await chrome.eval("window.scrollTo(0, 900)");
+      await chrome.eval("window.__delay = 400");
+      await click(".stc-dock .stc-pill--primary");
+      await Bun.sleep(120);
+      expect(await text(".lr-main .lr-lesson h1.title")).toBe("Introduction");
+      await until(async () => (await text(".lr-main .lr-lesson h1.title")) === "Watch the clip", "next lesson");
+      await chrome.eval("window.__delay = 0");
+      expect(await chrome.eval<number>("scrollY")).toBe(0);
+      await still("lesson next");
+
+      // module open
+      await chrome.eval(`[...document.querySelectorAll(".lr-mod-head")][1].click(), true`);
+      await still("module open");
+
+      // fold: straight from closed to open
+      const heights = await chrome.eval<number[]>(`(() => {
+        const d = document.querySelector("details.fold.transcript");
+        d.querySelector("summary").click();
+        const out = [], t0 = performance.now();
+        return new Promise((r) => { const f = () => { out.push(parseFloat(getComputedStyle(d, "::details-content").height)); if (performance.now() - t0 < 300) requestAnimationFrame(f); else r(out); }; f(); });
+      })()`);
+      expect(new Set(heights).size).toBe(1);
+      await still("fold");
+
+      // slide change: flips without animating
+      await until(() => chrome.eval<boolean>(`document.querySelector(".stage canvas[data-front]")?.dataset.page === "1"`), "deck");
+      await click('.deckbar [aria-label="Next slide"]');
+      await until(() => chrome.eval<boolean>(`document.querySelector(".stage canvas[data-front]").dataset.page === "2"`), "slide 2");
+      await still("slide");
+
+      // mark done (progress fills), menu, toast
+      await click(".stc-dock button[aria-pressed]");
+      await Bun.sleep(400);
+      await still("progress");
+      await click(".lr-switch-btn");
+      await until(() => exists(".lr-menu"), "menu");
+      await still("menu");
+      await chrome.send("Input.dispatchKeyEvent", { type: "keyDown", key: "Escape", code: "Escape", windowsVirtualKeyCode: 27 });
+      await chrome.send("Input.dispatchKeyEvent", { type: "keyUp", key: "Escape", code: "Escape", windowsVirtualKeyCode: 27 });
+      await open("/c/sample/m2-ex-add", ".stc-seg button");
+      await chrome.eval(`[...document.querySelectorAll(".stc-seg button")].find((b) => b.textContent === "Reading").click(), true`);
+      await until(() => exists(".lr-toast"), "toast");
+      await still("toast");
+      await until(async () => !(await exists(".lr-toast")), "toast removed", 4000);
+      expect((await m()).anims).toEqual([]);
+    } finally {
+      await setReducedMotion(false);
+    }
+  }, 60_000);
 });
diff --git a/packages/ui/src/tokens.css b/packages/ui/src/tokens.css
index 398b585..2ef20d3 100644
--- a/packages/ui/src/tokens.css
+++ b/packages/ui/src/tokens.css
@@ -97,4 +97,6 @@ code, pre { font-family: var(--mono); }
 @media (prefers-reduced-motion: reduce) {
   *, *::before, *::after { animation: none !important; transition: none !important; scroll-behavior: auto !important; }
   ::view-transition-group(*), ::view-transition-old(*), ::view-transition-new(*) { animation: none !important; }
+  /* separate rule: browsers without ::details-content drop only this one */
+  *::details-content { transition: none !important; }
 }
diff --git a/packages/ui/test/motion-css.test.ts b/packages/ui/test/motion-css.test.ts
index 8da11d1..07eefc5 100644
--- a/packages/ui/test/motion-css.test.ts
+++ b/packages/ui/test/motion-css.test.ts
@@ -88,6 +88,8 @@ test("prefers-reduced-motion turns off every animation, transition and view tran
   expect(block).toMatch(/::view-transition-group\(\*\)/);
   expect(block).toMatch(/::view-transition-old\(\*\)/);
   expect(block).toMatch(/::view-transition-new\(\*\)/);
+  // `*` does not reach ::details-content (fold heights), so it has its own rule.
+  expect(block).toMatch(/\*::details-content\s*\{\s*transition:\s*none !important/);
 });
 
 test("view transitions never take pointer input and keep the header and sidebar still", () => {