diff --git a/apps/web/src/components/ChatView.tsx b/apps/web/src/components/ChatView.tsx
index 6a021ef7e5e2..d5e9b29ba19c 100644
--- a/apps/web/src/components/ChatView.tsx
+++ b/apps/web/src/components/ChatView.tsx
@@ -433,7 +433,7 @@ import {
 import { environmentShell } from "../state/shell";
 import { ChatComposer, type ChatComposerHandle } from "./chat/ChatComposer";
 import { createPageScrollController, type PageScrollKey } from "./chat/pageScrollController";
-import { isTimelineScrollTarget } from "./chat/timelineScrollTarget";
+import { createUpwardScrollDetector, isTimelineScrollTarget } from "./chat/timelineScrollTarget";
 import { DraftHeroHeadline } from "./chat/DraftHeroHeadline";
 import { ExpandedImageDialog } from "./chat/ExpandedImageDialog";
 import { PullRequestThreadDialog } from "./PullRequestThreadDialog";
@@ -6621,6 +6621,22 @@ export default function ChatView(props: ChatViewProps) {
               break;
           }
         };
+        // Input inside an embedded frame reaches none of the listeners above,
+        // but its scroll still chains out to the timeline (#17056).
+        const scrolledUp = createUpwardScrollDetector({
+          top: scrollNode.scrollTop,
+          height: scrollNode.scrollHeight,
+        });
+        const handleScroll = () => {
+          if (
+            scrolledUp({ top: scrollNode.scrollTop, height: scrollNode.scrollHeight }) &&
+            timelineScrollModeRef.current !== "free-scrolling" &&
+            viewportIsAwayFromEnd()
+          ) {
+            handleManualNavigation();
+          }
+        };
+        scrollNode.addEventListener("scroll", handleScroll, { passive: true });
         scrollNode.addEventListener("wheel", handleWheel, {
           passive: true,
         });
@@ -6632,6 +6648,7 @@ export default function ChatView(props: ChatViewProps) {
         });
         document.addEventListener("keydown", handleKeyDown);
         removeListeners = () => {
+          scrollNode.removeEventListener("scroll", handleScroll);
           scrollNode.removeEventListener("wheel", handleWheel);
           scrollNode.removeEventListener("touchmove", handleTouchMove);
           scrollNode.removeEventListener("pointerdown", handlePointerDown);
diff --git a/apps/web/src/components/chat/timelineScrollTarget.test.ts b/apps/web/src/components/chat/timelineScrollTarget.test.ts
index 051f7785f5a3..b3c3a2c1f9fc 100644
--- a/apps/web/src/components/chat/timelineScrollTarget.test.ts
+++ b/apps/web/src/components/chat/timelineScrollTarget.test.ts
@@ -4,7 +4,7 @@ import {
   createComposerScrollGestureState,
   recordComposerScrollGestureEvent,
 } from "./composerScrollGesture";
-import { isTimelineScrollTarget } from "./timelineScrollTarget";
+import { createUpwardScrollDetector, isTimelineScrollTarget } from "./timelineScrollTarget";
 
 class ScrollElement extends EventTarget {
   scrollTop = 0;
@@ -140,3 +140,38 @@ describe("timeline scroll targets", () => {
     expect(record(content, 60, 14)).toBe(true);
   });
 });
+
+describe("createUpwardScrollDetector", () => {
+  it("reports a reader scrolling up, such as a scroll chained out of an embedded frame", () => {
+    const scrolledUp = createUpwardScrollDetector({ top: 6621, height: 7421 });
+    expect(scrolledUp({ top: 6471, height: 7421 })).toBe(true);
+    expect(scrolledUp({ top: 6321, height: 7421 })).toBe(true);
+  });
+
+  it("ignores moves toward the end and content growing while it follows", () => {
+    const scrolledUp = createUpwardScrollDetector({ top: 1000, height: 2000 });
+    expect(scrolledUp({ top: 1000, height: 2400 })).toBe(false);
+    expect(scrolledUp({ top: 1400, height: 2400 })).toBe(false);
+  });
+
+  it("ignores the offset clamping up when content above the end shrinks", () => {
+    const scrolledUp = createUpwardScrollDetector({ top: 1400, height: 2400 });
+    expect(scrolledUp({ top: 1100, height: 2100 })).toBe(false);
+    // The next upward move is measured from the clamped position.
+    expect(scrolledUp({ top: 900, height: 2100 })).toBe(true);
+  });
+
+  it("ignores subpixel jitter", () => {
+    const scrolledUp = createUpwardScrollDetector({ top: 500.5, height: 2000 });
+    expect(scrolledUp({ top: 500, height: 2000 })).toBe(false);
+    expect(scrolledUp({ top: 500.5, height: 2000 })).toBe(false);
+    expect(scrolledUp({ top: 500, height: 2000 })).toBe(false);
+  });
+
+  it("adds up a slow scroll that moves under a pixel per event", () => {
+    const scrolledUp = createUpwardScrollDetector({ top: 500, height: 2000 });
+    expect(scrolledUp({ top: 499.5, height: 2000 })).toBe(false);
+    expect(scrolledUp({ top: 499, height: 2000 })).toBe(false);
+    expect(scrolledUp({ top: 498.5, height: 2000 })).toBe(true);
+  });
+});
diff --git a/apps/web/src/components/chat/timelineScrollTarget.ts b/apps/web/src/components/chat/timelineScrollTarget.ts
index da87e9a7036d..0c00fda095a5 100644
--- a/apps/web/src/components/chat/timelineScrollTarget.ts
+++ b/apps/web/src/components/chat/timelineScrollTarget.ts
@@ -29,3 +29,28 @@ export function isTimelineScrollTarget(
   }
   return true;
 }
+
+/**
+ * Reports, for each timeline scroll, whether the reader moved it up. A scroll
+ * that chains out of an embedded frame (an HTML render or MCP app) reaches the
+ * timeline with no wheel, touch, or pointer event it can see. Following only
+ * moves toward the end, and shrinking content can only pull the offset up by
+ * clamping, so an upward move while the content kept its height is the reader.
+ */
+export function createUpwardScrollDetector(initial: { top: number; height: number }) {
+  let baseline = initial;
+  return (next: { top: number; height: number }) => {
+    // Shrinking content or a move toward the end starts a new measurement.
+    if (next.height < baseline.height || next.top > baseline.top) {
+      baseline = next;
+      return false;
+    }
+    if (next.top < baseline.top - 1) {
+      baseline = next;
+      return true;
+    }
+    // A slow scroll can move under a pixel per event; let those moves add up.
+    baseline = { top: baseline.top, height: next.height };
+    return false;
+  };
+}