diff --git a/src/components/layout/app-shell/AppShell.test.tsx b/src/components/layout/app-shell/AppShell.test.tsx
index 3d5fc1f47..30ae3640c 100644
--- a/src/components/layout/app-shell/AppShell.test.tsx
+++ b/src/components/layout/app-shell/AppShell.test.tsx
@@ -286,6 +286,42 @@ describe("AppShell controlled sidebar open", () => {
     expect(screen.queryByLabelText("Open agent")).not.toBeInTheDocument();
   });
 
+  it("observes the body height on a reload-restore open, so the background paints", async () => {
+    // Regression for "sometimes the sidebar has no background": the body's
+    // height feeds AgentSidebarHeader's single full-window shape (the fill).
+    // On the reload-restore path open stays true from mount while the width
+    // seeds 0 → >0, so isOpen NEVER transitions. The body height observer must
+    // still attach when the body node mounts, or agentBodyH is stuck at 0, the
+    // header drops to its body-less cap, and the panel paints with no
+    // background. A callback ref (not a [isOpen]-keyed effect) attaches on the
+    // actual mount; pre-fix, that mount was missed and the body went unobserved.
+    const observed: Element[] = [];
+    class MockResizeObserver {
+      constructor(_cb: ResizeObserverCallback) {}
+      observe(el: Element) {
+        observed.push(el);
+      }
+      unobserve() {}
+      disconnect() {}
+    }
+    vi.stubGlobal("ResizeObserver", MockResizeObserver);
+    try {
+      render(
+        <AppShell open onOpenChange={() => {}}>
+          content
+        </AppShell>,
+      );
+      await act(async () => {
+        await Promise.resolve();
+      });
+      const body = document.querySelector(".flex-1.min-h-0.flex.flex-col");
+      expect(body).not.toBeNull();
+      expect(observed).toContain(body);
+    } finally {
+      vi.unstubAllGlobals();
+    }
+  });
+
   it("paints closed (FAB shown) when open is false", () => {
     render(
       <AppShell open={false} onOpenChange={() => {}}>
diff --git a/src/components/layout/app-shell/AppShell.tsx b/src/components/layout/app-shell/AppShell.tsx
index 56f1c2c7b..e44c1b323 100644
--- a/src/components/layout/app-shell/AppShell.tsx
+++ b/src/components/layout/app-shell/AppShell.tsx
@@ -1,4 +1,10 @@
-import { useState, useRef, useEffect, type ReactNode } from "react";
+import {
+  useState,
+  useRef,
+  useEffect,
+  useCallback,
+  type ReactNode,
+} from "react";
 import { cn } from "@/utils/cn";
 import type { ComponentMeta } from "@/types/component-meta";
 import { IconButton } from "@/components/ui/actions/icon-button/IconButton";
@@ -309,7 +315,7 @@ function AppShellInner({
   // the window-minus-header height the AgentSidebarHeader needs to draw the
   // whole window as ONE Notch (rounded body + active-tab notch, single fill) —
   // killing the header↔body cream seam that aliases on fractional-DPI.
-  const agentBodyRef = useRef<HTMLDivElement>(null);
+  const agentBodyObserverRef = useRef<ResizeObserver | null>(null);
   const [agentBodyH, setAgentBodyH] = useState(0);
 
   useEffect(() => {
@@ -327,20 +333,27 @@ function AppShellInner({
   // (uncontrolled — behavior unchanged when `open` is undefined).
   const isOpen = open ?? sidebarWidth > 0;
 
-  // Track the agent body's own height (window height MINUS the 40px header) so
+  // Measure the agent body's own height (window height MINUS the 40px header) so
   // the single window shape always matches it through opens, drag-resize, and
-  // viewport changes. Re-runs when the sidebar mounts/unmounts (isOpen) so the
-  // observer attaches once the body element exists.
-  useEffect(() => {
-    const el = agentBodyRef.current;
-    if (!el) return;
-    if (typeof ResizeObserver === "undefined") return;
+  // viewport changes. This is a CALLBACK REF, not a ref + [isOpen] effect,
+  // because the body node mounts on `isOpen && sidebarWidth > 0` — a condition
+  // that can flip true WITHOUT isOpen changing: the reload-restore path holds
+  // open=true while the width seeds 0 → >0 (and the async width GET lands the
+  // same way). An [isOpen]-keyed effect misses that mount, so the observer never
+  // attaches, agentBodyH stays 0, the header falls back to its body-less cap, and
+  // the sidebar paints with NO background until an unrelated close+reopen toggles
+  // isOpen. A callback ref fires exactly when the node attaches/detaches, so it
+  // can never desync from the actual mount.
+  const setAgentBodyEl = useCallback((el: HTMLDivElement | null) => {
+    agentBodyObserverRef.current?.disconnect();
+    agentBodyObserverRef.current = null;
+    if (!el || typeof ResizeObserver === "undefined") return;
     const observer = new ResizeObserver((entries) => {
       setAgentBodyH(entries[0].contentRect.height);
     });
     observer.observe(el);
-    return () => observer.disconnect();
-  }, [isOpen]);
+    agentBodyObserverRef.current = observer;
+  }, []);
 
   const isOverlaying =
     isOpen &&
@@ -571,7 +584,7 @@ function AppShellInner({
                   (flex/min-h-0) and drop bg-on-background + rounded-2xl so the shape
                   shows through with no abutting seam. The inner scroller and input
                   carry no opaque bg of their own, so this is the only fill to strip. */}
-              <div ref={agentBodyRef} className="flex-1 min-h-0 flex flex-col">
+              <div ref={setAgentBodyEl} className="flex-1 min-h-0 flex flex-col">
                 <div className="flex-1 overflow-y-auto overflow-x-hidden p-4">
                   {/* The host's chat surface when wired; otherwise the
                       personalized empty-state opener under the brand logo