diff --git a/packages/app/e2e/support/helpers/agent-timeline-gate.ts b/packages/app/e2e/support/helpers/agent-timeline-gate.ts
index dd50ccfa3d..408bd8af24 100644
--- a/packages/app/e2e/support/helpers/agent-timeline-gate.ts
+++ b/packages/app/e2e/support/helpers/agent-timeline-gate.ts
@@ -575,20 +575,25 @@ export async function holdAssistantStream(page: Page, agentId: string) {
     });
   });
 
+  const forwardUntil = async (done: (text: string) => boolean): Promise<void> => {
+    while (!done(forwardedText)) {
+      const frame = pending.shift();
+      if (!frame) {
+        await new Promise<void>((resolve) => {
+          notifyFrame = resolve;
+        });
+        continue;
+      }
+      forwardedText += frame.text;
+      frame.forward();
+    }
+  };
+
   return {
     waitForInitialTimeline: () => initialTimeline,
+    showUntil: (text: string) => forwardUntil((forwarded) => forwarded.includes(text)),
     async showThrough(prefix: string): Promise<void> {
-      while (forwardedText.length < prefix.length) {
-        const frame = pending.shift();
-        if (!frame) {
-          await new Promise<void>((resolve) => {
-            notifyFrame = resolve;
-          });
-          continue;
-        }
-        forwardedText += frame.text;
-        frame.forward();
-      }
+      await forwardUntil((text) => text.length >= prefix.length);
       if (forwardedText !== prefix) {
         throw new Error(
           `Stream did not stop at ${JSON.stringify(prefix)}: ${JSON.stringify(forwardedText)}`,
diff --git a/packages/app/e2e/support/helpers/timeline-scroll-smoothness.ts b/packages/app/e2e/support/helpers/timeline-scroll-smoothness.ts
index dbbbd8e7a3..60db71b25e 100644
--- a/packages/app/e2e/support/helpers/timeline-scroll-smoothness.ts
+++ b/packages/app/e2e/support/helpers/timeline-scroll-smoothness.ts
@@ -3,6 +3,7 @@ import { createServer } from "node:http";
 import { expect, type Page, type TestInfo } from "@playwright/test";
 import { seedMockAgentWorkspace, openAgentRoute, type MockAgentWorkspace } from "./mock-agent";
 import { openAgentTimeline, expectTimelinePromptVisible } from "./timeline-pagination";
+import { holdAssistantStream } from "./agent-timeline-gate";
 
 export const scrollCadences = [
   { name: "slow", delta: 160, intervalMs: 50, steps: 1000 },
@@ -68,6 +69,7 @@ export async function withStreamingReader(run: (agent: MockAgentWorkspace) => Pr
 }
 
 export async function holdParagraphThroughCompletion(page: Page, agent: MockAgentWorkspace) {
+  const stream = await holdAssistantStream(page, agent.agentId);
   await openAgentRoute(page, agent);
   await page.getByRole("textbox", { name: "Message agent...", exact: true }).waitFor();
   const scroll = page.getByTestId("agent-chat-scroll");
@@ -85,9 +87,15 @@ export async function holdParagraphThroughCompletion(page: Page, agent: MockAgen
     return result;
   });
   await agent.client.sendAgentMessage(agent.agentId, "Stream the mixed response.");
+  await stream.showUntil("Long section 12\n");
   await page
     .getByRole("heading", { name: "Long section 12", exact: true })
     .waitFor({ state: "attached" });
+  // Exercise a reader whose producer has already finished; delivery must be gated.
+  await agent.client.waitForFinish(agent.agentId, 30_000);
+  await expect(page.getByTestId("mermaid-viewport")).toHaveCount(2);
+  await expect(page.getByTestId("mermaid-viewport").nth(0)).toHaveCSS("opacity", "1");
+  await expect(page.getByTestId("mermaid-viewport").nth(1)).toHaveCSS("opacity", "1");
   const maxFollowGap = await following.evaluate((result) => {
     result.stopped = true;
     return result.maxGap;
@@ -146,6 +154,7 @@ export async function holdParagraphThroughCompletion(page: Page, agent: MockAgen
   await expect(
     page.getByTestId("scroll-to-bottom-button").filter({ visible: true }).first(),
   ).toBeVisible();
+  stream.release();
   await page.waitForFunction((result) => result.done, recording);
   return { ...(await recording.jsonValue()), maxFollowGap };
 }