diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index f5b2f29990c86..7b5366915d387 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -4668,11 +4668,22 @@ export default function ChatView(props: ChatViewProps) {
             modelStrip.offsetHeight + Number.parseFloat(getComputedStyle(modelStrip).marginTop),
           )
         : 0;
+    const noticeStack = composerOverlayElement?.querySelector<HTMLElement>(
+      '[data-composer-banner-stack="true"]',
+    );
+    const expandedNoticesHeight = noticeStack
+      ? Math.max(
+          0,
+          noticeStack.getBoundingClientRect().height -
+            Number.parseFloat(getComputedStyle(noticeStack).minHeight),
+        )
+      : 0;
     const nextInset = resolveComposerTimelineInset({
       currentInset: composerTimelineInsetRef.current,
-      overlayHeight: nextHeight,
+      overlayHeight: height,
       isResting,
       restingOnlyHeight,
+      expandedNoticesHeight,
     });
     if (composerTimelineInsetRef.current !== nextInset) {
       composerTimelineInsetRef.current = nextInset;
diff --git a/apps/web/src/components/chat/ComposerBannerStack.tsx b/apps/web/src/components/chat/ComposerBannerStack.tsx
index dd9d7c570db37..8e410d90e648d 100644
--- a/apps/web/src/components/chat/ComposerBannerStack.tsx
+++ b/apps/web/src/components/chat/ComposerBannerStack.tsx
@@ -121,6 +121,7 @@ export function ComposerBannerStack({ className, items, attachedAbove }: Compose
       {hasStack ? (
         <div
           ref={noticesRef}
+          data-composer-banner-stack="true"
           data-chat-composer-collapsed-controls="true"
           className={cn("relative z-20 min-h-3", stackExpanded && "z-50")}
           onPointerEnter={(event) => {
diff --git a/apps/web/src/components/composerFooterLayout.test.ts b/apps/web/src/components/composerFooterLayout.test.ts
index f913c04391819..ef021a7dc013f 100644
--- a/apps/web/src/components/composerFooterLayout.test.ts
+++ b/apps/web/src/components/composerFooterLayout.test.ts
@@ -140,6 +140,35 @@ describe("resolveComposerTimelineInset", () => {
       resolveComposerTimelineInset({ currentInset: 0, overlayHeight: 60, isResting: true }),
     ).toBe(60 + COMPOSER_RESTING_EXPANSION_MIN_PX);
   });
+
+  it("keeps the timeline reservation stable through notice expansion and collapse", () => {
+    const collapsed = resolveComposerTimelineInset({
+      currentInset: 0,
+      overlayHeight: 172,
+      isResting: false,
+    });
+    const expanded = resolveComposerTimelineInset({
+      currentInset: collapsed,
+      overlayHeight: 532,
+      isResting: false,
+      expandedNoticesHeight: 360,
+    });
+    expect(expanded).toBe(collapsed);
+    const resting = resolveComposerTimelineInset({
+      currentInset: expanded,
+      overlayHeight: 470,
+      isResting: true,
+      expandedNoticesHeight: 360,
+    });
+    expect(resting).toBe(collapsed);
+    expect(
+      resolveComposerTimelineInset({
+        currentInset: resting,
+        overlayHeight: 110,
+        isResting: true,
+      }),
+    ).toBe(collapsed);
+  });
 });
 
 describe("shouldUseRestingComposerLayout", () => {
diff --git a/apps/web/src/components/composerFooterLayout.ts b/apps/web/src/components/composerFooterLayout.ts
index e0dce39603e01..5ae971f19a344 100644
--- a/apps/web/src/components/composerFooterLayout.ts
+++ b/apps/web/src/components/composerFooterLayout.ts
@@ -59,16 +59,20 @@ export function resolveComposerTimelineInset(input: {
   overlayHeight: number;
   isResting: boolean;
   restingOnlyHeight?: number;
+  expandedNoticesHeight?: number;
 }): number {
+  // Expanded notices temporarily cover the timeline. Remove their growth before
+  // rounding so fractional measurements reserve exactly the collapsed height.
+  const overlayHeight = Math.ceil(input.overlayHeight - (input.expandedNoticesHeight ?? 0));
   return input.isResting
     ? Math.max(
         input.currentInset,
-        input.overlayHeight +
+        overlayHeight +
           (input.currentInset === 0
             ? COMPOSER_RESTING_EXPANSION_MIN_PX - (input.restingOnlyHeight ?? 0)
             : 0),
       )
-    : input.overlayHeight;
+    : overlayHeight;
 }
 
 export function overlayComposerIsResting(input: {