diff --git a/src/components/code-session-rail.tsx b/src/components/code-session-rail.tsx
index 3b7d35f56..21f86a5c2 100644
--- a/src/components/code-session-rail.tsx
+++ b/src/components/code-session-rail.tsx
@@ -64,6 +64,7 @@ export function CodeSessionRail({
   onExpand,
 }: CodeSessionRailProps) {
   const groups = groupCodeRailSessions(sessions);
+  const openRailClassName = onExpand ? "py-2" : "overflow-y-auto py-2";
   const newButton = open && onNewSession ? (
     <div className="px-2 pb-1">
       <button
@@ -93,7 +94,7 @@ export function CodeSessionRail({
   return (
     <nav
       aria-label="Coding sessions"
-      className={`flex h-full min-h-0 flex-col ${open ? "overflow-y-auto py-2" : "items-center gap-1"}`}
+      className={`flex h-full min-h-0 flex-col ${open ? openRailClassName : "items-center gap-1"}`}
     >
       {newButton}
       {groups.map((group) => (
diff --git a/src/components/code-surface-mode.test.ts b/src/components/code-surface-mode.test.ts
index d7c8fefae..e16774216 100644
--- a/src/components/code-surface-mode.test.ts
+++ b/src/components/code-surface-mode.test.ts
@@ -509,6 +509,16 @@ assert.match(
   /onClick=\{\(\) => \{\s*onExpand\?\.\(\);\s*onSelect\(row\.id\);/,
   "collapsed session buttons expand before selection",
 );
+assert.match(
+  rail,
+  /const openRailClassName = onExpand \? "py-2" : "overflow-y-auto py-2";/,
+  "SurfaceRail-hosted sessions defer scrolling to the shared rail while the narrow standalone list remains scrollable",
+);
+assert.match(
+  rail,
+  /open \? openRailClassName : "items-center gap-1"/,
+  "the session navigation applies the layout-specific open-state scrolling contract",
+);
 
 assert.match(
   composer,