diff --git a/CHANGELOG.md b/CHANGELOG.md
index d7082ef9f..e768cb7e5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -36,6 +36,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   disabled-path test against a second no-thebe fixture served on its own port,
   proving the toggle is gated on the project opting in ([#98](https://github.com/QuantEcon/quantecon-theme.mystmd/pull/98)).
 
+### Fixed
+- Contents sidebar no longer flashes open on page load. On static builds the
+  first paint can happen before `app.css` applies, leaving the panel in flow and
+  fully visible; it then animated itself shut over 300ms because the transition
+  predated the stylesheet. The inlined critical CSS now parks the panel
+  off-screen on that first frame, and the transition is withheld until after
+  mount ([#123](https://github.com/QuantEcon/quantecon-theme.mystmd/pull/123)).
+
 ## [2.2.0] - 2026-07-16
 
 > Headline: fancy ordered lists — `(a)` / `(i)` / `B)` markers from the QuantEcon
diff --git a/app/components/ContentsSidebar.tsx b/app/components/ContentsSidebar.tsx
index 8e3fb270b..a22a86815 100644
--- a/app/components/ContentsSidebar.tsx
+++ b/app/components/ContentsSidebar.tsx
@@ -12,6 +12,7 @@ import {
 import { useSidebarHeight } from '@myst-theme/site';
 import classNames from 'classnames';
 import { slugToUrl } from 'myst-common';
+import { useEffect, useState } from 'react';
 
 type StrictHeading = Omit<Heading, 'level'> & { level: number };
 type HeadingGroup = StrictHeading[];
@@ -43,8 +44,22 @@ 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();
   const config = useSiteManifest();
   const project = useProjectManifest();
   const top = useThemeTop();
@@ -84,13 +99,28 @@ export function ContentsSidebar() {
     <div
       ref={toc}
       className={classNames(
+        // Stable hook for the inline critical CSS in app/root.tsx, which parks
+        // this panel off-screen on the very first paint — before app.css lands.
+        'qe-contents-sidebar',
         'fixed top-0 left-0',
         'w-[350px] lg:w-[250px] 2xl:w-[350px]',
         'h-screen w-[250px] z-[20] pt-[40px] pb-[90px] px-9',
         'bg-qetoolbar-light dark:bg-qetoolbar-dark ',
         'border-r-[1px] border-qetoolbar-border',
-        'transition-all duration-300 ease-in-out',
         'overflow-y-auto',
+        // Belt and braces, not the primary guard. The critical CSS above is
+        // what actually prevents the flash; because both states are a -100%
+        // translate there is nothing for a transition to interpolate, so
+        // removing this gate does not by itself reintroduce it (measured).
+        //
+        // It is kept because the transition is only ever wanted in response to
+        // a click: withholding it until after mount means any correction made
+        // when app.css lands is applied instantly rather than animated, which
+        // keeps this component correct even if the critical rule is later
+        // changed or dropped.
+        //
+        // `transform` (not `all`) so only the slide animates, on the compositor.
+        mounted && 'transition-transform duration-300 ease-in-out',
         { 'translate-x-0': open, '-translate-x-full': !open }
       )}
       style={{ top: '50px' }}
diff --git a/app/root.tsx b/app/root.tsx
index c844c541e..e616998af 100644
--- a/app/root.tsx
+++ b/app/root.tsx
@@ -55,6 +55,11 @@ export const meta: V2_MetaFunction<typeof loader> = ({ data }) => {
  * arrives. This keeps the inline block from overriding the live cascade despite
  * being emitted after <Links /> in the document head.
  *
+ * Because these rules carry no specificity, every property set here MUST also
+ * be declared by the real stylesheet, otherwise it can never be overridden.
+ * (E.g. parking the nav panel off-screen with `visibility:hidden` would stick
+ * forever, since no Tailwind class sets `visibility` — hence the transform.)
+ *
  * Keep the values in sync with their sources of truth:
  *   - font stack:    tailwind.config.js  -> theme.extend.fontFamily.sans
  *   - grid columns:  tailwind.config.js  -> theme.extend.gridTemplateColumns
@@ -65,6 +70,15 @@ export const meta: V2_MetaFunction<typeof loader> = ({ data }) => {
  *                    page background; the inner content panel uses `qepage-dark`
  *                    #222, see app/components/Page.tsx — intentionally not set here
  *                    since these rules target <body>.)
+ *   - nav panel:     app/components/ContentsSidebar.tsx -> `.qe-contents-sidebar`
+ *                    (only the class name needs to stay in sync; see below)
+ *
+ * The nav-panel rule deliberately sets no width. `translateX(-100%)` resolves
+ * against the element's own border box, so its right edge lands at `left + W -
+ * W` = 0 for **any** width W — it is off-screen before app.css arrives and
+ * stays off-screen after, even though the resolved width differs between the
+ * two (350/250/350 across the base/lg/2xl bands). `position:fixed` is set so
+ * the panel does not push the article down while it waits.
  */
 const CRITICAL_CSS = `
 :where(html){font-family:"Source Sans 3",sans-serif}
@@ -74,6 +88,7 @@ const CRITICAL_CSS = `
 :where(.simple-center-grid){display:grid;grid-template-columns:[screen-start] 1fr [body-start] minmax(300px,800px) [body-end] 1fr [screen-end]}
 :where(.simple-center-grid) > *{grid-column:body-start / body-end}
 @media (min-width:1280px){:where(.simple-center-grid){grid-template-columns:[screen-start] 1fr 200px 20px [body-start] 800px [body-end] 20px [margin-start] 200px [margin-end] 1fr [screen-end]}}
+:where(.qe-contents-sidebar){position:fixed;left:0;transform:translateX(-100%)}
 `;
 
 export const links: LinksFunction = () => {
diff --git a/tests/visual/fouc.spec.ts b/tests/visual/fouc.spec.ts
index 684155523..5e50a620a 100644
--- a/tests/visual/fouc.spec.ts
+++ b/tests/visual/fouc.spec.ts
@@ -9,6 +9,11 @@ import { test, expect, type Page, type Route } from "@playwright/test";
  * grid collapsed to `display: block`. The fix inlines critical CSS into `<head>`
  * (see `app/root.tsx`), which parses synchronously and styles that first paint.
  *
+ * The same unstyled frame also exposed the contents sidebar: `-translate-x-full`
+ * does nothing until app.css lands, so the panel painted in-flow and visible,
+ * then slid shut once the stylesheet arrived — the "menu opens on load" report.
+ * It is covered here too, since the cause and the guard are the same.
+ *
  * This test makes the failure mode deterministic by **aborting all external
  * stylesheets**, so the only styling that can reach the page is the inline
  * `<style>`. If the inline critical CSS regresses, the "styled first paint"
@@ -21,6 +26,22 @@ import { test, expect, type Page, type Route } from "@playwright/test";
 
 const PAGE = "/";
 
+/**
+ * Slack, in CSS px, allowed on the parked nav panel's right edge.
+ *
+ * `translateX(-100%)` puts that edge at exactly 0 in theory, but the panel is
+ * unstyled at this point, so its width is shrink-to-fit and lands on a
+ * fractional value (135.171875px in WebKit here). WebKit snaps the painted
+ * translate to a whole device pixel while leaving the border-box width
+ * fractional, so the measured edge comes back at +0.171875 rather than 0 — the
+ * exact remainder varies with the intrinsic width, hence with platform font
+ * metrics. A strict `> 0` test therefore passes on the ubuntu CI runner and
+ * fails on macOS for the same, correct, markup.
+ *
+ * 1px is well inside "not visible" and well outside any snapping remainder.
+ */
+const OFFSCREEN_EPS = 1;
+
 async function isolateInlineCss(page: Page, { stripCritical = false } = {}) {
   await page.route("**/*", async (route: Route) => {
     const req = route.request();
@@ -49,6 +70,7 @@ async function firstPaintState(page: Page) {
   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
@@ -59,6 +81,18 @@ async function firstPaintState(page: Page) {
     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),
     };
@@ -72,9 +106,15 @@ 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);
-    // The two reported FOUC symptoms must be absent on first paint:
+    // The reported FOUC symptoms must be absent on first paint:
     expect(state.gridDisplay).toBe("grid"); // grid not collapsed to block
     expect(state.bodyFont).toMatch(/Source Sans 3/); // sans, not the serif default
+    expect(
+      state.sidebarRight,
+      "`.qe-contents-sidebar` not found — the hook the critical CSS targets was renamed or removed"
+    ).not.toBeNull();
+    // Nav panel parked off-screen (right edge at 0, modulo the sub-pixel slack).
+    expect(state.sidebarRight).toBeLessThan(OFFSCREEN_EPS);
   });
 
   test("control: removing the inline critical CSS reproduces the FOUC", async ({ page }) => {
@@ -86,5 +126,14 @@ test.describe("FOUC guard (WebKit) — inline critical CSS styles the first pain
     expect(state.appliedExternal).toBe(false);
     expect(state.gridDisplay).toBe("block");
     expect(state.bodyFont).not.toMatch(/Source Sans 3/);
+    // Without the inline rule the nav panel lays out in-flow and fully visible —
+    // this is the "menu flashes open on load" symptom. Measured at ~1272px of a
+    // 1280px viewport, so the margin over OFFSCREEN_EPS is three orders of
+    // magnitude: the two states are never in danger of being confused.
+    expect(
+      state.sidebarRight,
+      "`.qe-contents-sidebar` not found — the hook the critical CSS targets was renamed or removed"
+    ).not.toBeNull();
+    expect(state.sidebarRight).toBeGreaterThan(OFFSCREEN_EPS);
   });
 });