diff --git a/CHANGELOG.md b/CHANGELOG.md
index 31d1be0a7..2f3f7be09 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- The Contents sidebar's width below `lg` no longer depends on Tailwind's
+  rule-emission order. The panel carried two unprefixed width utilities on the
+  same element — `w-[350px]` from the responsive line and a stale `w-[250px]`
+  on the next class string — which land in the same layer at the same
+  specificity, so the base band resolved to 350px only because Tailwind sorts
+  arbitrary values as strings and `[250px]` sorts before `[350px]`. Change the
+  base width to anything sorting earlier and the stale class silently took
+  over, with no error to explain why. The stale utility is gone, leaving
+  `w-[350px] lg:w-[250px] 2xl:w-[350px]` as the single source of the width.
+  Nothing moves: the resolved widths stay 350/250/350 across the base/`lg`/`2xl`
+  bands ([#130](https://github.com/QuantEcon/quantecon-theme.mystmd/issues/130)).
+
 ## [2.3.0] - 2026-08-20
 
 > Headline: lecture pages gain **in-page live compute** (a JupyterLite/Pyodide
diff --git a/app/components/ContentsSidebar.tsx b/app/components/ContentsSidebar.tsx
index a22a86815..3b7f1d856 100644
--- a/app/components/ContentsSidebar.tsx
+++ b/app/components/ContentsSidebar.tsx
@@ -103,8 +103,12 @@ export function ContentsSidebar() {
         // this panel off-screen on the very first paint — before app.css lands.
         'qe-contents-sidebar',
         'fixed top-0 left-0',
+        // Every band of the panel width lives on this one line. A second
+        // unprefixed width anywhere in these class strings lands in the same
+        // layer at the same specificity, so which one wins below `lg` would be
+        // decided by Tailwind's emission order rather than by intent (#130).
         'w-[350px] lg:w-[250px] 2xl:w-[350px]',
-        'h-screen w-[250px] z-[20] pt-[40px] pb-[90px] px-9',
+        'h-screen z-[20] pt-[40px] pb-[90px] px-9',
         'bg-qetoolbar-light dark:bg-qetoolbar-dark ',
         'border-r-[1px] border-qetoolbar-border',
         'overflow-y-auto',