diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf6f1b854..17bcc5b8b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -37,6 +37,35 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   directory rather than assuming one. A production build now emits **zero**
   absolute asset URLs and all 78 references resolve
   ([#150](https://github.com/QuantEcon/quantecon-theme.mystmd/issues/150)).
+- The sidebar toggle icons no longer animate their first-paint correction.
+  #123 stopped the contents panel flashing open on static-build loads; the
+  button that drives it had the identical shape and was untouched. Both lucide
+  icons carry `transition-all` with visibility driven by opacity, and on the
+  pre-`app.css` frame none of `absolute`, `opacity-0` or `opacity-100` exists
+  while lucide still emits real `width`/`height` attributes — so both icons
+  paint in flow at full opacity, and when the stylesheet lands `position` snaps
+  but `opacity` 1 → 0 *animates*, fading a close icon out of the toolbar on
+  every navigation. The transition is now withheld until after mount, making
+  that correction instant, and narrowed from `transition-all` to the three
+  properties that were ever meant to move (opacity, transform for
+  `hover:scale-110`, and colour so the icons still ease across a dark-mode
+  toggle). "Back to top" in the page margin had the same shape and is covered
+  too. The `useMounted` hook #123 introduced is hoisted to `app/hooks/` and
+  shared by all three sites
+  ([#127](https://github.com/QuantEcon/quantecon-theme.mystmd/issues/127)).
+- The WebKit FOUC guard no longer goes red for reasons unrelated to the critical
+  CSS. Hydration currently fails on every page load (React #418/#423), and the
+  recovery re-render restores the inlined critical `<style>` about 195ms in —
+  after the control test has deliberately stripped it from the served HTML to
+  prove the guard is meaningful. The measurement used to run from the test after
+  `domcontentloaded`, leaving roughly a 150ms budget, so on a loaded runner all
+  three control assertions flipped at once. It now samples from an init script
+  that fires in-page on `DOMContentLoaded`, before hydration is even scheduled.
+  Both cases also assert their own preconditions — that the strip actually
+  matched, and whether the inline block is present as sampled — so a reshaped
+  `CRITICAL_CSS` reports as a stale strip pattern instead of as three confusing
+  failures about grid layout. The underlying hydration failure remains open in
+  [#126](https://github.com/QuantEcon/quantecon-theme.mystmd/issues/126).
 
 ## [2.3.1] - 2026-08-26
 
diff --git a/app/components/ContentsSidebar.tsx b/app/components/ContentsSidebar.tsx
index a22a86815..453263a83 100644
--- a/app/components/ContentsSidebar.tsx
+++ b/app/components/ContentsSidebar.tsx
@@ -12,7 +12,7 @@ import {
 import { useSidebarHeight } from '@myst-theme/site';
 import classNames from 'classnames';
 import { slugToUrl } from 'myst-common';
-import { useEffect, useState } from 'react';
+import useMounted from '~/hooks/useMounted';
 
 type StrictHeading = Omit<Heading, 'level'> & { level: number };
 type HeadingGroup = StrictHeading[];
@@ -44,19 +44,6 @@ function Section({ group }: { group: HeadingGroup }) {
   );
 }
 
-/**
- * True only after the component has mounted on the client.
- *
- * Both the server render and the first (hydrating) client render return
- * `false`, so the markup matches and React does not warn; the effect then
- * flips it on the frame after hydration.
- */
-function useMounted() {
-  const [mounted, setMounted] = useState(false);
-  useEffect(() => setMounted(true), []);
-  return mounted;
-}
-
 export function ContentsSidebar() {
   const [open] = useNavOpen();
   const mounted = useMounted();
diff --git a/app/components/Outline.tsx b/app/components/Outline.tsx
index a48030fdc..03966a406 100644
--- a/app/components/Outline.tsx
+++ b/app/components/Outline.tsx
@@ -1,17 +1,24 @@
 import { useBaseurl, useLinkProvider, withBaseurl } from '@myst-theme/providers';
 import { useHeaders } from '@myst-theme/site';
 import classNames from 'classnames';
+import useMounted from '~/hooks/useMounted';
 import useScroll from '~/hooks/useScroll';
 
 export function BackToTop() {
   const isScrolled = useScroll(80);
+  const mounted = useMounted();
   const Link = useLinkProvider();
   return (
     <div className="fixed bottom-0 left-0 right-0 col-screen not-prose simple-center-grid grid-gap">
       <div className="relative col-margin">
         <p
           className={classNames(
-            'absolute bottom-0 font-semibold text-md z-[1000] transition-opacity ease-in-out duration-300',
+            'absolute bottom-0 font-semibold text-md z-[1000]',
+            // Withheld until after mount: before app.css lands `opacity-0` does
+            // not exist, so this paints visible and would then *fade* out when
+            // the stylesheet arrives rather than simply never having been
+            // there. Same mechanism as ContentsSidebar and SidebarToggle.
+            mounted && 'transition-opacity ease-in-out duration-300',
             {
               'opacity-100': isScrolled,
               'opacity-0': !isScrolled,
diff --git a/app/components/toolbar/SidebarToggle.tsx b/app/components/toolbar/SidebarToggle.tsx
index 7d342c976..035425e3e 100644
--- a/app/components/toolbar/SidebarToggle.tsx
+++ b/app/components/toolbar/SidebarToggle.tsx
@@ -1,28 +1,48 @@
 import { useNavOpen } from '@myst-theme/providers';
 import classNames from 'classnames';
 import { Menu, X } from 'lucide-react';
+import useMounted from '~/hooks/useMounted';
 
 export function SidebarToggle() {
   const [open, setOpen] = useNavOpen();
+  const mounted = useMounted();
+
+  // Both icons are always rendered, stacked by `absolute` and cross-faded by
+  // opacity.
+  //
+  // The transition is withheld until after mount for the same reason as the
+  // panel this button drives (see ContentsSidebar.tsx). On a static build the
+  // first paint happens before app.css applies, so neither `absolute` nor
+  // `opacity-0` exists yet and both icons paint in flow at full opacity —
+  // lucide emits real width/height attributes, so they have size without any
+  // CSS. When app.css lands `position` snaps, since it is not animatable, but
+  // `opacity` 1 → 0 would *animate*: a close icon fading out of the toolbar on
+  // every navigation. Withholding the transition until mount makes that
+  // correction instant and leaves the cross-fade for real clicks.
+  //
+  // Narrowed from `transition-all`, which animated every property app.css
+  // changes on that frame. The three named here are the ones that were ever
+  // meant to move: opacity for the cross-fade, transform for `hover:scale-110`,
+  // and colour so the icons still ease across a dark-mode toggle.
+  const iconClasses = (visible: boolean) =>
+    classNames('absolute hover:scale-110', mounted && 'transition-[opacity,transform,color] duration-300 ease-in-out', {
+      'opacity-100': visible,
+      'opacity-0': !visible,
+    });
+
   return (
     <button
       className="relative flex items-center w-6 h-6 cursor-pointer opacity-90"
       onClick={() => setOpen(!open)}
     >
       <X
-        className={classNames('absolute transition-all duration-300 ease-in-out hover:scale-110', {
-          'opacity-0': !open,
-          'opacity-100': open,
-        })}
+        className={iconClasses(open)}
         width={24}
         height={24}
         aria-label="Hide table of contents"
       />
       <Menu
-        className={classNames('absolute transition-all duration-300 ease-in-out hover:scale-110', {
-          'opacity-100': !open,
-          'opacity-0': open,
-        })}
+        className={iconClasses(!open)}
         width={24}
         height={24}
         aria-label="Show table of contents"
diff --git a/app/hooks/useMounted.tsx b/app/hooks/useMounted.tsx
new file mode 100644
index 000000000..354e287c2
--- /dev/null
+++ b/app/hooks/useMounted.tsx
@@ -0,0 +1,23 @@
+import { useEffect, useState } from 'react';
+
+/**
+ * True only after the component has mounted on the client.
+ *
+ * Both the server render and the first (hydrating) client render return
+ * `false`, so the markup matches and React does not warn; the effect then
+ * flips it on the frame after hydration.
+ *
+ * Used to withhold CSS transitions until the stylesheet has landed. On a static
+ * build the first paint happens before `app.css` applies, so any state the
+ * stylesheet then corrects — an `opacity-0`, a `-translate-x-full` — would
+ * *animate* into place if the `transition-*` class predated it. Gating the
+ * transition on this makes that correction instant, and reserves the animation
+ * for genuine, post-hydration state changes.
+ */
+const useMounted = () => {
+  const [mounted, setMounted] = useState(false);
+  useEffect(() => setMounted(true), []);
+  return mounted;
+};
+
+export default useMounted;
diff --git a/tests/visual/fouc.spec.ts b/tests/visual/fouc.spec.ts
index e999da775..e9de69b35 100644
--- a/tests/visual/fouc.spec.ts
+++ b/tests/visual/fouc.spec.ts
@@ -42,7 +42,18 @@ const PAGE = "/";
  */
 const OFFSCREEN_EPS = 1;
 
+/**
+ * Aborts external stylesheets, optionally stripping the inline critical block.
+ *
+ * Returns a `strippedCritical` probe rather than trusting the regex to have
+ * matched. If `CRITICAL_CSS` is ever reshaped so the pattern stops matching,
+ * the control would otherwise run against a fully-styled page and fail on its
+ * *assertions* — reporting "expected block, received grid", which points at the
+ * critical CSS rather than at the stale pattern that is actually at fault.
+ * Asserting this probe turns that into a failure that names itself.
+ */
 async function isolateInlineCss(page: Page, { stripCritical = false } = {}) {
+  const probe = { strippedCritical: false };
   await page.route("**/*", async (route: Route) => {
     const req = route.request();
 
@@ -54,49 +65,96 @@ async function isolateInlineCss(page: Page, { stripCritical = false } = {}) {
     // Control: serve the document with the inlined critical <style> removed.
     if (req.resourceType() === "document" && stripCritical) {
       const response = await route.fetch();
-      const body = (await response.text()).replace(
+      const served = await response.text();
+      const body = served.replace(
         /<style>[^<]*:where\(\.simple-center-grid\)[^<]*<\/style>/g,
         "<!-- critical CSS removed for control -->",
       );
+      if (body !== served) probe.strippedCritical = true;
       return route.fulfill({ response, body });
     }
 
     return route.continue();
   });
+  return probe;
 }
 
-async function firstPaintState(page: Page) {
+/**
+ * Key under which the in-page sampler parks its measurement on `window`.
+ */
+const SAMPLE_KEY = "__qeFirstPaint";
+
+type FirstPaint = {
+  gridDisplay: string;
+  bodyFont: string;
+  sidebarRight: number | null;
+  appliedExternal: boolean;
+  criticalInline: boolean;
+};
+
+/**
+ * Measure at first paint, from inside the page, before React hydrates.
+ *
+ * The measurement used to run from the test after `domcontentloaded`, which
+ * left roughly a 150ms budget before hydration. That is not enough: hydration
+ * currently fails on every load (React #418/#423, tracked in #126), and the
+ * recovery re-render puts the critical `<style>` back — around 195ms in
+ * sampling. The control strips that block from the served HTML precisely to
+ * prove the guard is meaningful, so when React restored it the control's
+ * assertions all flipped at once and `fouc-guard` went red for reasons that had
+ * nothing to do with the critical CSS.
+ *
+ * Sampling from an init script closes that window: it runs on
+ * `DOMContentLoaded`, in-page and synchronously, and it reads only the DOM. The
+ * ordering is empirical rather than guaranteed — Remix v1 emits its entry as an
+ * inline `type="module" async` script whose imports must resolve before
+ * `entry.client.tsx` even schedules `requestIdleCallback`/`setTimeout` for
+ * `hydrateRoot`, and WebKit dispatches DOMContentLoaded at end of parse — but
+ * that ordering is what the #126 timings show, with the earliest observed
+ * restoration an order of magnitude later than the sample.
+ *
+ * Note this deliberately survives a *fixed* #126: a control that strips the
+ * inline block guarantees a hydration mismatch by construction, so no repair to
+ * the hydration failure itself could make a post-hydration sample safe here.
+ */
+async function firstPaintState(page: Page): Promise<FirstPaint> {
+  await page.addInitScript((key) => {
+    document.addEventListener(
+      "DOMContentLoaded",
+      () => {
+        const grid = document.querySelector(".simple-center-grid");
+        const sidebar = document.querySelector(".qe-contents-sidebar");
+        const applied = Array.from(document.styleSheets).filter((sheet) => {
+          try {
+            return !!sheet.cssRules && sheet.cssRules.length > 0; // applied, not pending
+          } catch {
+            return false; // cross-origin / not yet loaded
+          }
+        });
+        const sample = {
+          gridDisplay: grid ? getComputedStyle(grid).display : "(absent)",
+          bodyFont: getComputedStyle(document.body).fontFamily,
+          sidebarRight: sidebar ? sidebar.getBoundingClientRect().right : null,
+          appliedExternal: applied.some((sheet) => !!sheet.href),
+          // Whether the inline block is present in the document as sampled —
+          // the state the assertions below are actually about.
+          criticalInline: Array.from(document.querySelectorAll("style")).some((el) =>
+            (el.textContent ?? "").includes(":where(.simple-center-grid)"),
+          ),
+        };
+        (window as unknown as Record<string, unknown>)[key] = sample;
+      },
+      { once: true },
+    );
+  }, SAMPLE_KEY);
+
   await page.goto(PAGE, { waitUntil: "domcontentloaded" });
-  await page.waitForSelector(".simple-center-grid", { timeout: 5000 }).catch(() => {});
-  return page.evaluate(() => {
-    const grid = document.querySelector(".simple-center-grid");
-    const sidebar = document.querySelector(".qe-contents-sidebar");
-    const applied = Array.from(document.styleSheets).filter((sheet) => {
-      try {
-        return !!sheet.cssRules && sheet.cssRules.length > 0; // applied, not pending
-      } catch {
-        return false; // cross-origin / not yet loaded
-      }
-    });
-    return {
-      gridDisplay: grid ? getComputedStyle(grid).display : "(absent)",
-      bodyFont: getComputedStyle(document.body).fontFamily,
-      // The nav panel is closed on load, so it must start parked off-screen to
-      // the left: its right edge at (or left of) the viewport's left edge. If
-      // any real width of it is on-screen here, the menu visibly flashes open.
-      //
-      // Reported as the measured edge rather than a boolean so the assertions
-      // can carry a sub-pixel tolerance — see `OFFSCREEN_EPS` below.
-      //
-      // `null` when the element is missing, and the guards below reject it
-      // explicitly: `null` numerically coerces to 0, which would *pass* the
-      // off-screen check, so a renamed hook would slip through the main test
-      // and surface as a confusing failure in the control instead.
-      sidebarRight: sidebar ? sidebar.getBoundingClientRect().right : null,
-      // null href == an inline <style>; any string == an external sheet that applied
-      appliedExternal: applied.some((sheet) => !!sheet.href),
-    };
-  });
+  await page.waitForFunction((key) => !!(window as any)[key], SAMPLE_KEY, { timeout: 5000 });
+  const state = await page.evaluate<FirstPaint, string>(
+    (key) => (window as any)[key],
+    SAMPLE_KEY,
+  );
+  return state;
 }
 
 test.describe("FOUC guard (WebKit) — inline critical CSS styles the first paint", () => {
@@ -106,6 +164,10 @@ test.describe("FOUC guard (WebKit) — inline critical CSS styles the first pain
 
     // No external sheet applied, so anything styled below comes from inline CSS.
     expect(state.appliedExternal).toBe(false);
+    expect(
+      state.criticalInline,
+      "the inline critical <style> is missing from the page as served — app/root.tsx no longer inlines CRITICAL_CSS"
+    ).toBe(true);
     // The reported FOUC symptoms must be absent on first paint:
     expect(state.gridDisplay).toBe("grid"); // grid not collapsed to block
     // Head of the stack, not just a substring: "Source Sans 3 Variable" (the
@@ -125,9 +187,20 @@ test.describe("FOUC guard (WebKit) — inline critical CSS styles the first pain
   });
 
   test("control: removing the inline critical CSS reproduces the FOUC", async ({ page }) => {
-    await isolateInlineCss(page, { stripCritical: true });
+    const probe = await isolateInlineCss(page, { stripCritical: true });
     const state = await firstPaintState(page);
 
+    // Preconditions first, so a stale strip pattern reports as itself rather
+    // than as three confusing assertion failures about the critical CSS.
+    expect(
+      probe.strippedCritical,
+      "the strip pattern matched nothing in the served HTML — CRITICAL_CSS was reshaped and the regex in isolateInlineCss needs updating"
+    ).toBe(true);
+    expect(
+      state.criticalInline,
+      "the inline critical <style> is present despite the strip — it was restored before the sample (see #126)"
+    ).toBe(false);
+
     // With no CSS at all, the page is unstyled — this proves the guard above is
     // meaningful (the external abort really does strip styling).
     expect(state.appliedExternal).toBe(false);