Resume cached timelines without replaying their tail (#3329)

* perf(timeline): resume cached history from its exact range

Warm launches were repainting cached history and then downloading the same bounded tail to rediscover that nothing changed. Persist synchronization authority only when the complete current canonical window can be encoded losslessly; otherwise retain the existing display-only fallback.

* test(timeline): hide replica setup behind E2E helper

* fix(timeline): bound persisted resume overflow

* test(timeline): cover workspace eviction catch-up order

* test(timeline): read persisted range from IndexedDB

packages/app/e2e/browser/agent-message-submission.spec.ts

@@ -30,6 +30,12 @@ import { buildHostWorkspaceRoute } from "@/utils/host-routes";
 import { delayBrowserAgentCreatedStatus } from "../support/helpers/new-workspace";
 import { installDaemonWebSocketGate } from "../support/helpers/daemon-websocket-gate";
 import { selectModel } from "../support/helpers/app";
+import { observeTimelineSubscriptions } from "../support/helpers/timeline-delivery";
+import {
+  expectResumeOverflowFallsBackToOneTail,
+  rememberTimelineRequestCounts,
+} from "../support/helpers/timeline-resume";
+import { workspaceDeckEntryLocator } from "../support/helpers/workspace-ui";
 import {
   scrollTimelineToNewestLoadedEdge,
   scrollTimelineUntilOlderHistoryIsReachable,
@@ -336,6 +342,64 @@ async function expectInterruptedTurnOrderAfterReconnect(
   }
 }
 
+async function expectHiddenStreamingSubmissionOrderAfterWorkspaceEviction(
+  page: Page,
+  testInfo: { workerIndex: number },
+): Promise<void> {
+  const subscriptions = observeTimelineSubscriptions(page);
+  const gate = await installDaemonWebSocketGate(page);
+  const target = await seedMockAgentWorkspace({
+    repoPrefix: `submission-hidden-stream-${testInfo.workerIndex}-`,
+    title: "Hidden streaming submission",
+    model: "ten-second-stream",
+  });
+  const evictionAgents = await Promise.all(
+    Array.from({ length: 3 }, (_unused, index) =>
+      seedMockAgentWorkspace({
+        repoPrefix: `submission-workspace-eviction-${testInfo.workerIndex}-${index}-`,
+        title: `Workspace eviction ${index + 1}`,
+      }),
+    ),
+  );
+  const prompt = "Keep this hidden image prompt before its streaming output.";
+  const targetDeckEntry = workspaceDeckEntryLocator(page, getServerId(), target.workspaceId);
+
+  try {
+    await openAgentRoute(page, target);
+    await expectComposerVisible(page);
+    await subscriptions.waitForSubscribedAgents([target.agentId]);
+
+    const userMessageCount = gate.getAgentStreamItemCount("user_message");
+    gate.setAgentStreamSuppressed(true);
+    const promptRow = await submitMessageWithImage(page, prompt);
+    await gate.waitForAgentStreamItem("user_message", userMessageCount + 1);
+
+    for (const evictionAgent of evictionAgents) {
+      await openAgentRoute(page, evictionAgent);
+      await expectComposerVisible(page);
+    }
+    await expect(targetDeckEntry).toHaveCount(0);
+    await subscriptions.waitForSubscribedAgents([evictionAgents[2]!.agentId]);
+    gate.setAgentStreamSuppressed(false);
+
+    await target.client.waitForFinish(target.agentId, 30_000);
+    const requestsBeforeReturn = rememberTimelineRequestCounts(gate);
+    await openAgentRoute(page, target);
+    await expectComposerVisible(page);
+    await subscriptions.waitForSubscribedAgents([target.agentId]);
+
+    const response = page.getByText("(end of synthetic stream)", { exact: true }).last();
+    await expect(promptRow).toBeVisible();
+    await expect(response).toBeVisible();
+    await expectRenderedBefore(promptRow, response);
+    expectResumeOverflowFallsBackToOneTail(gate, requestsBeforeReturn);
+  } finally {
+    gate.setAgentStreamSuppressed(false);
+    gate.restore();
+    await Promise.all([...evictionAgents.map((agent) => agent.cleanup()), target.cleanup()]);
+  }
+}
+
 async function expectCompletedSubmissionClearsAfterMissedRunningTransition(
   page: Page,
   testInfo: { workerIndex: number },
@@ -1013,6 +1077,13 @@ test.describe("Agent message submission", () => {
     await expectInterruptedTurnOrderAfterReconnect(page, testInfo);
   });
 
+  test("keeps a streaming hidden submission before its output after workspace eviction", async ({
+    page,
+  }, testInfo) => {
+    test.setTimeout(120_000);
+    await expectHiddenStreamingSubmissionOrderAfterWorkspaceEviction(page, testInfo);
+  });
+
   test("clears an attachment-only submission when canonical history arrives after a missed running transition", async ({
     page,
   }, testInfo) => {