fix(app): stabilize session timeline layout continuity (#34533)

packages/app/e2e/regression/session-timeline-accessibility.spec.ts

@@ -0,0 +1,22 @@
+import { expect, test } from "@playwright/test"
+import { assistantMessage, setupTimeline, shell, userMessage } from "../performance/timeline-stability/fixture"
+
+test("space activates a focused timeline button instead of scrolling", async ({ page }) => {
+  const shellID = "prt_space_button_shell"
+  await setupTimeline(page, {
+    messages: [userMessage(), assistantMessage([shell(shellID, "completed", lines(5))])],
+    settings: { shellToolPartsExpanded: false },
+    reducedMotion: true,
+  })
+  const scroller = page.locator(".scroll-view__viewport", { has: page.locator("[data-timeline-row]") })
+  const trigger = page.locator(`[data-timeline-part-id="${shellID}"] [data-slot="collapsible-trigger"]`)
+  await trigger.focus()
+  const before = await scroller.evaluate((element) => element.scrollTop)
+  await trigger.press("Space")
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  expect(await scroller.evaluate((element) => element.scrollTop)).toBe(before)
+})
+
+function lines(count: number) {
+  return Array.from({ length: count }, (_, index) => `line ${index + 1}`).join("\n")
+}

packages/app/e2e/regression/session-timeline-context-resize.spec.ts

@@ -1,6 +1,13 @@
 import { expect, test, type Page } from "@playwright/test"
 import { mockOpenCodeServer } from "../utils/mock-server"
 import { expectAppVisible, expectSessionTitle } from "../utils/waits"
+import {
+  analyzeVisualObservations,
+  defineVisualRegions,
+  startVisualProbe,
+  stopVisualProbe,
+  visualPlan,
+} from "../utils/visual-stability"
 
 const directory = "C:/OpenCode/ContextResizeRegression"
 const projectID = "proj_context_resize_regression"
@@ -36,6 +43,82 @@ test.describe("regression: session timeline context group resize", () => {
     expect(visibleOverlap).toEqual([])
     expect(samples.at(-1)?.expanded).toBe("true")
   })
+
+  test("paints a stable exploring to explored transition", async ({ page }) => {
+    const events: { directory: string; payload: Record<string, unknown> }[] = []
+    await page.setViewportSize({ width: 1400, height: 900 })
+    await mockServer(page, events, [
+      ...Array.from({ length: 8 }, (_, index) => turn(index, false)).flat(),
+      ...turn(10, true, "running"),
+    ])
+    await configurePage(page)
+
+    await page.goto(`/${base64Encode(directory)}/session/${sessionID}`)
+    await expectSessionTitle(page, title)
+    const devtools = await page.context().newCDPSession(page)
+    await devtools.send("Emulation.setCPUThrottlingRate", { rate: 4 })
+    const context = page.locator(`[data-timeline-part-ids="${contextIDs.join(",")}"]`).first()
+    await expectAppVisible(context)
+    await expect(context.locator('[data-component="tool-status-title"]')).toHaveAttribute("aria-label", "Exploring")
+
+    const contextSelector = `[data-timeline-part-ids="${contextIDs.join(",")}"]`
+    const regions = defineVisualRegions({
+      status: {
+        selector: `${contextSelector} [data-component="tool-status-title"]`,
+        opacitySelectors: ['[data-slot="tool-status-active"]', '[data-slot="tool-status-done"]'],
+      },
+      context: { selector: contextSelector, closest: '[data-timeline-row="AssistantPart"]' },
+      following: {
+        selector: `[data-timeline-part-id="${followingTextID}"]`,
+        closest: '[data-timeline-row="AssistantPart"]',
+      },
+    })
+    await startVisualProbe(page, regions)
+    for (const [index, delay] of [120, 350, 80, 500].entries()) {
+      events.push({
+        directory,
+        payload: {
+          type: "message.part.updated",
+          properties: {
+            part: contextTool(
+              contextIDs[index]!,
+              id("msg_assistant", 10),
+              ["read", "glob", "grep", "list"][index]!,
+              [
+                { filePath: "src/recent-a.ts" },
+                { path: directory, pattern: "**/*.ts" },
+                { path: directory, pattern: "Explored" },
+                { path: "src" },
+              ][index]!,
+            ),
+          },
+        },
+      })
+      await page.waitForTimeout(delay)
+    }
+
+    await expect(context.locator('[data-component="tool-status-title"]')).toHaveAttribute("aria-label", "Explored")
+    await page.waitForTimeout(700)
+    const trace = await stopVisualProbe<keyof typeof regions>(page)
+    const labels = trace.samples
+      .map((sample) => sample.regions.status?.label)
+      .filter((value): value is string => !!value)
+      .filter((value, index, all) => value !== all[index - 1])
+    const issues = analyzeVisualObservations(
+      trace.samples,
+      visualPlan(regions, [
+        { type: "required", regions: ["context", "following"] },
+        { type: "opacity", regions: "all" },
+        { type: "continuity", regions: "all" },
+        { type: "motion", regions: "all" },
+        { type: "label-stability", regions: "all" },
+        { type: "flow", regions: ["context", "following"] },
+      ]),
+    )
+
+    expect(labels).toEqual(["Exploring", "Explored"])
+    expect(issues, JSON.stringify(trace.samples, null, 2)).toEqual([])
+  })
 })
 
 async function configurePage(page: Page) {
@@ -128,7 +211,7 @@ async function sampleExpansion(page: Page) {
   )
 }
 
-function turn(index: number, target: boolean): Message[] {
+function turn(index: number, target: boolean, status: "running" | "completed" = "completed"): Message[] {
   const userID = id("msg_user", index)
   const assistantID = id("msg_assistant", index)
   return [
@@ -163,10 +246,22 @@ function turn(index: number, target: boolean): Message[] {
       },
       parts: target
         ? [
-            contextTool(contextIDs[0]!, assistantID, "read", { filePath: "src/recent-a.ts", offset: 0, limit: 120 }),
-            contextTool(contextIDs[1]!, assistantID, "glob", { path: directory, pattern: "**/*.ts" }),
-            contextTool(contextIDs[2]!, assistantID, "grep", { path: directory, pattern: "Explored", include: "*.ts" }),
-            contextTool(contextIDs[3]!, assistantID, "list", { path: "src" }),
+            contextTool(
+              contextIDs[0]!,
+              assistantID,
+              "read",
+              { filePath: "src/recent-a.ts", offset: 0, limit: 120 },
+              status,
+            ),
+            contextTool(contextIDs[1]!, assistantID, "glob", { path: directory, pattern: "**/*.ts" }, status),
+            contextTool(
+              contextIDs[2]!,
+              assistantID,
+              "grep",
+              { path: directory, pattern: "Explored", include: "*.ts" },
+              status,
+            ),
+            contextTool(contextIDs[3]!, assistantID, "list", { path: "src" }, status),
             {
               id: followingTextID,
               sessionID,
@@ -188,7 +283,13 @@ function turn(index: number, target: boolean): Message[] {
   ]
 }
 
-function contextTool(partID: string, messageID: string, tool: string, input: Record<string, unknown>) {
+function contextTool(
+  partID: string,
+  messageID: string,
+  tool: string,
+  input: Record<string, unknown>,
+  status: "running" | "completed" = "completed",
+) {
   return {
     id: partID,
     sessionID,
@@ -197,7 +298,7 @@ function contextTool(partID: string, messageID: string, tool: string, input: Rec
     callID: `call_${partID}`,
     tool,
     state: {
-      status: "completed",
+      status,
       input,
       output: `Completed ${tool}.\n${"detail line\n".repeat(8)}`,
       title: input.filePath || input.path || input.pattern || "completed",
@@ -207,13 +308,19 @@ function contextTool(partID: string, messageID: string, tool: string, input: Rec
   }
 }
 
-async function mockServer(page: Page) {
+async function mockServer(
+  page: Page,
+  events: { directory: string; payload: Record<string, unknown> }[] = [],
+  fixtureMessages = messages,
+) {
   await mockOpenCodeServer(page, {
     directory,
     project: project(),
     provider: provider(),
     sessions: [session()],
-    pageMessages: () => ({ items: messages }),
+    pageMessages: () => ({ items: fixtureMessages }),
+    events: () => events.splice(0, 1),
+    eventRetry: 50,
   })
 }
 

packages/app/e2e/regression/session-timeline-context-state.spec.ts

@@ -0,0 +1,31 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  partUpdated,
+  setupTimeline,
+  toolPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+test("preserves a collapsed context group through count and status updates", async ({ page }) => {
+  const ids = ["prt_closed_01_read", "prt_closed_02_glob"]
+  const inputs = {
+    read: { filePath: "src/a.ts", offset: 0, limit: 120 },
+    glob: { path: ".", pattern: "**/*.ts" },
+  }
+  const timeline = await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage(
+        [toolPart(ids[0]!, "read", "running", inputs.read), toolPart(ids[1]!, "glob", "running", inputs.glob)],
+        { completed: false },
+      ),
+    ],
+  })
+  const group = page.locator(`[data-timeline-part-ids="${ids.join(",")}"]`)
+  const trigger = group.locator('[data-slot="collapsible-trigger"]')
+  await expect(trigger).toHaveAttribute("aria-expanded", "false")
+  await timeline.send(partUpdated(toolPart(ids[0]!, "read", "completed", inputs.read)), 100)
+  await timeline.send(partUpdated(toolPart(ids[1]!, "glob", "completed", inputs.glob)), 300)
+  await expect(trigger).toHaveAttribute("aria-expanded", "false")
+})

packages/app/e2e/regression/session-timeline-file-projection.spec.ts

@@ -0,0 +1,52 @@
+import { expect, test } from "@playwright/test"
+import { assistantMessage, setupTimeline, toolPart, userMessage } from "../performance/timeline-stability/fixture"
+
+test("renders completed write content", async ({ page }) => {
+  const id = "prt_file_projection_write"
+  await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage([
+        toolPart(id, "write", "completed", { filePath: "src/write.ts", content: "export const written = true\n" }),
+      ]),
+    ],
+    settings: { editToolPartsExpanded: true },
+  })
+
+  await expect(page.locator(`[data-timeline-part-id="${id}"] [data-component="write-content"]`)).toBeVisible()
+})
+
+test("renders a completed single-file patch", async ({ page }) => {
+  const id = "prt_file_projection_single_patch"
+  await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage([
+        toolPart(
+          id,
+          "apply_patch",
+          "completed",
+          { files: ["src/a.ts"] },
+          {
+            metadata: {
+              files: [
+                {
+                  filePath: "src/a.ts",
+                  relativePath: "src/a.ts",
+                  type: "update",
+                  additions: 1,
+                  deletions: 1,
+                  before: "export const value = 1\n",
+                  after: "export const value = 2\n",
+                },
+              ],
+            },
+          },
+        ),
+      ]),
+    ],
+    settings: { editToolPartsExpanded: true },
+  })
+
+  await expect(page.locator(`[data-timeline-part-id="${id}"] [data-component="apply-patch-file-diff"]`)).toBeVisible()
+})

packages/app/e2e/regression/session-timeline-file-state.spec.ts

@@ -0,0 +1,94 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  partUpdated,
+  setupTimeline,
+  toolPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+test("updates edit diagnostics without resetting manual collapse state", async ({ page }) => {
+  const editID = "prt_diagnostics_edit"
+  const base = editPart(editID, [])
+  const timeline = await setupTimeline(page, {
+    messages: [userMessage(), assistantMessage([base])],
+    settings: { editToolPartsExpanded: true },
+  })
+  const trigger = page.locator(`[data-timeline-part-id="${editID}"] [data-slot="collapsible-trigger"]`).first()
+  await trigger.click()
+  await expect(trigger).toHaveAttribute("aria-expanded", "false")
+  await timeline.send(
+    partUpdated(editPart(editID, [diagnostic("First failure", 2), diagnostic("Second failure", 4)])),
+    300,
+  )
+  await expect(trigger).toHaveAttribute("aria-expanded", "false")
+  await timeline.send(partUpdated(editPart(editID, [])), 300)
+  await expect(trigger).toHaveAttribute("aria-expanded", "false")
+})
+
+test("preserves nested patch file state through outer collapse and reopen", async ({ page }) => {
+  const patchID = "prt_nested_patch"
+  const files = [patchFile("src/a.ts", "update"), patchFile("src/b.ts", "add"), patchFile("src/old.ts", "delete")]
+  await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage([
+        toolPart(
+          patchID,
+          "apply_patch",
+          "completed",
+          { files: files.map((file) => file.filePath) },
+          { metadata: { files } },
+        ),
+      ]),
+    ],
+    settings: { editToolPartsExpanded: true },
+  })
+  const wrapper = page.locator(`[data-timeline-part-id="${patchID}"]`)
+  const outer = wrapper.locator('[data-slot="collapsible-trigger"]').first()
+  const deleted = wrapper.locator('[data-scope="apply-patch"] [data-type="delete"]')
+  await deleted.getByRole("button").click()
+  await expect(deleted.getByRole("button")).toHaveAttribute("aria-expanded", "true")
+  await outer.click()
+  await expect(outer).toHaveAttribute("aria-expanded", "false")
+  await outer.click()
+  await expect(outer).toHaveAttribute("aria-expanded", "true")
+  await expect(deleted.getByRole("button")).toHaveAttribute("aria-expanded", "true")
+})
+
+function patchFile(filePath: string, type: "add" | "update" | "delete") {
+  return {
+    filePath,
+    relativePath: filePath,
+    type,
+    additions: type === "delete" ? 0 : 4,
+    deletions: type === "add" ? 0 : 3,
+    before: type === "add" ? undefined : source(false),
+    after: type === "delete" ? undefined : source(true),
+  }
+}
+
+function editPart(id: string, diagnostics: Record<string, unknown>[]) {
+  return toolPart(
+    id,
+    "edit",
+    "completed",
+    { filePath: "src/edit.ts" },
+    {
+      metadata: {
+        filediff: { file: "src/edit.ts", additions: 1, deletions: 1, before: source(false), after: source(true) },
+        diagnostics,
+      },
+    },
+  )
+}
+
+function diagnostic(message: string, line: number) {
+  return { message, severity: 1, range: { start: { line, character: 0 }, end: { line, character: 2 } } }
+}
+
+function source(changed: boolean) {
+  return Array.from({ length: 12 }, (_, index) => `export const value${index} = ${changed ? index + 1 : index}\n`).join(
+    "",
+  )
+}

packages/app/e2e/regression/session-timeline-lifecycle-state.spec.ts

@@ -0,0 +1,94 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  completedAssistantInfo,
+  messageUpdated,
+  partUpdated,
+  reasoningPart,
+  setupTimeline,
+  shell,
+  status,
+  textPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+for (const expanded of [false, true]) {
+  test(`preserves shell user intent from a ${expanded ? "expanded" : "collapsed"} default`, async ({ page }) => {
+    const id = `prt_shell_default_${expanded}`
+    const timeline = await setupTimeline(page, {
+      messages: [userMessage(), assistantMessage([shell(id, "completed", lines(3))])],
+      settings: { shellToolPartsExpanded: expanded },
+    })
+    const trigger = page.locator(`[data-timeline-part-id="${id}"] [data-slot="collapsible-trigger"]`)
+    await expect(trigger).toHaveAttribute("aria-expanded", String(expanded))
+    await trigger.click()
+    await expect(trigger).toHaveAttribute("aria-expanded", String(!expanded))
+
+    await timeline.send(partUpdated(shell(id, "completed", lines(6))), 180)
+    await timeline.send(partUpdated(textPart(`prt_sibling_${expanded}`, "Sibling content")), 180)
+    await timeline.send(status("busy"), 100)
+    await timeline.send(status("idle"), 250)
+    await expect(trigger).toHaveAttribute("aria-expanded", String(!expanded))
+  })
+}
+
+test("transitions thinking and hidden reasoning through busy to idle", async ({ page }) => {
+  const reasoningID = "prt_reasoning_hidden"
+  const assistant = assistantMessage([reasoningPart(reasoningID, "## Inspecting stability")], { completed: false })
+  const timeline = await setupTimeline(page, {
+    messages: [userMessage(), assistant],
+    settings: { showReasoningSummaries: false },
+    cpuRate: 4,
+  })
+  await timeline.send(status("busy"), 150)
+
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toBeVisible()
+  await expect(page.getByText("Inspecting stability", { exact: true })).toBeVisible()
+  await expect(page.locator(`[data-timeline-part-id="${reasoningID}"]`)).toHaveCount(0)
+  await timeline.send(partUpdated(shell("prt_reasoning_shell", "running")), 160)
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toBeVisible()
+  await timeline.send(partUpdated(shell("prt_reasoning_shell", "completed", "done")), 180)
+  await timeline.send(messageUpdated(completedAssistantInfo(assistant.info)), 100)
+  await timeline.send(status("idle"), 300)
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toHaveCount(0)
+  await expect(page.locator(`[data-timeline-part-id="${reasoningID}"]`)).toHaveCount(0)
+})
+
+test("moves busy through retry and recovery to final idle content", async ({ page }) => {
+  const assistant = assistantMessage([], { completed: false })
+  const timeline = await setupTimeline(page, {
+    messages: [
+      userMessage(undefined, {
+        summary: {
+          diffs: [
+            {
+              file: "src/retry.ts",
+              additions: 1,
+              deletions: 1,
+              patch: "@@ -1 +1 @@\n-export const retry = false\n+export const retry = true",
+            },
+          ],
+        },
+      }),
+      assistant,
+    ],
+  })
+  await timeline.send(status("busy"), 140)
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toBeVisible()
+  await expect(page.locator('[data-timeline-row="DiffSummary"]')).toHaveCount(0)
+  await timeline.send(status("retry"), 180)
+  await expect(page.locator('[data-timeline-row="Retry"]')).toBeVisible()
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toHaveCount(0)
+  await timeline.send(status("busy", 2), 180)
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toBeVisible()
+  await timeline.send(partUpdated(textPart("prt_recovered", "Recovered response")), 140)
+  await timeline.send(messageUpdated(completedAssistantInfo(assistant.info)), 100)
+  await timeline.send(status("idle"), 350)
+  await expect(page.locator('[data-timeline-row="Retry"]')).toHaveCount(0)
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toHaveCount(0)
+  await expect(page.locator('[data-timeline-row="DiffSummary"]')).toBeVisible()
+})
+
+function lines(count: number) {
+  return Array.from({ length: count }, (_, index) => `line ${index + 1}`).join("\n")
+}

packages/app/e2e/regression/session-timeline-locale-projection.spec.ts

@@ -0,0 +1,25 @@
+import { expect, test } from "@playwright/test"
+import { assistantMessage, setupTimeline, toolPart, userMessage } from "../performance/timeline-stability/fixture"
+
+for (const profile of [
+  { locale: "de", label: "Erkundet" },
+  { locale: "ar", label: "تم الاستكشاف" },
+] as const) {
+  test(`projects translated context status in ${profile.locale}`, async ({ page }) => {
+    const ids = [`prt_locale_${profile.locale}_01_read`, `prt_locale_${profile.locale}_02_glob`]
+    await setupTimeline(page, {
+      messages: [
+        userMessage(),
+        assistantMessage([
+          toolPart(ids[0]!, "read", "completed", { filePath: "src/a.ts" }),
+          toolPart(ids[1]!, "glob", "completed", { path: ".", pattern: "**/*.ts" }),
+        ]),
+      ],
+      locale: profile.locale,
+    })
+
+    const group = page.locator(`[data-timeline-part-ids="${ids.join(",")}"]`)
+    await expect(group.locator('[data-component="tool-status-title"]')).toHaveAttribute("aria-label", profile.label)
+    await expect(page.locator("html")).toHaveAttribute("lang", profile.locale)
+  })
+}

packages/app/e2e/regression/session-timeline-projection.spec.ts

@@ -0,0 +1,284 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  setupTimeline,
+  status,
+  toolPart,
+  userMessage,
+  userText,
+  type PartSeed,
+} from "../performance/timeline-stability/fixture"
+
+test.describe("session timeline projection", () => {
+  test("renders every admitted tool family and hides timeline-only exclusions", async ({ page }) => {
+    const parts = [
+      toolPart("prt_01_read", "read", "completed", { filePath: "src/a.ts" }),
+      toolPart("prt_02_glob", "glob", "completed", { path: ".", pattern: "**/*.ts" }),
+      toolPart("prt_03_grep", "grep", "completed", { path: ".", pattern: "value" }),
+      toolPart("prt_04_list", "list", "completed", { path: "src" }),
+      toolPart("prt_webfetch", "webfetch", "completed", { url: "https://example.com" }),
+      toolPart(
+        "prt_websearch",
+        "websearch",
+        "completed",
+        { query: "timeline stability" },
+        { output: "https://example.com/result" },
+      ),
+      toolPart("prt_task", "task", "completed", { description: "Inspect timeline", subagent_type: "explore" }),
+      toolPart(
+        "prt_bash",
+        "bash",
+        "completed",
+        { command: "printf stable" },
+        { output: "stable", title: "printf stable" },
+      ),
+      editPart("prt_edit"),
+      toolPart("prt_write", "write", "completed", { filePath: "src/new.ts", content: "export const stable = true\n" }),
+      patchPart("prt_patch"),
+      toolPart("prt_todo", "todowrite", "completed", { todos: [{ content: "Hidden", status: "pending" }] }),
+      toolPart(
+        "prt_question",
+        "question",
+        "completed",
+        { questions: [{ question: "Keep stable?", header: "Stability", options: [] }] },
+        { metadata: { answers: [["Yes"]] } },
+      ),
+      toolPart("prt_skill", "skill", "completed", { name: "stability" }),
+      toolPart("prt_custom", "custom_mcp_tool", "completed", { target: "timeline", count: 2 }),
+    ]
+    await setupTimeline(page, { messages: [userMessage(), assistantMessage(parts)] })
+
+    await expect(
+      page.locator('[data-timeline-part-ids="prt_01_read,prt_02_glob,prt_03_grep,prt_04_list"]'),
+    ).toBeVisible()
+    for (const id of [
+      "prt_webfetch",
+      "prt_websearch",
+      "prt_task",
+      "prt_bash",
+      "prt_edit",
+      "prt_write",
+      "prt_patch",
+      "prt_question",
+      "prt_skill",
+      "prt_custom",
+    ]) {
+      await expect(page.locator(`[data-timeline-part-id="${id}"]`).first(), id).toBeVisible()
+    }
+    await expect(page.locator('[data-timeline-part-id="prt_todo"]')).toHaveCount(0)
+  })
+
+  test("projects gaps, dividers, assistant parts, and errors together", async ({ page }) => {
+    const firstUser = userMessage(
+      [
+        userText("The user made the following comment regarding lines 4 through 8 of src/a.ts: Keep this stable", {
+          id: "prt_comment",
+          synthetic: true,
+          metadata: {
+            opencodeComment: {
+              path: "src/a.ts",
+              selection: { startLine: 4, startChar: 0, endLine: 8, endChar: 0 },
+              comment: "Keep this stable",
+            },
+          },
+        }),
+        userText("Continue after the comment", { id: "prt_visible_user" }),
+      ],
+      { summary: { diffs: Array.from({ length: 11 }, (_, index) => summaryDiff(index)) } },
+    )
+    const aborted = assistantMessage(
+      [
+        { id: "prt_before_abort", type: "text", text: "Before interruption" },
+        { id: "prt_compaction", type: "compaction", auto: true },
+      ],
+      {
+        id: "msg_1001_assistant_aborted",
+        error: { name: "MessageAbortedError", data: { message: "Stopped" } },
+      },
+    )
+    const failed = assistantMessage([{ id: "prt_after_abort", type: "text", text: "After interruption" }], {
+      id: "msg_1002_assistant_failed",
+      error: {
+        name: "APIError",
+        data: {
+          message: JSON.stringify({ error: { type: "provider_error", message: "Visible provider failure" } }),
+          isRetryable: false,
+        },
+      },
+      created: 1700000003000,
+    })
+    const nextUser = userMessage([userText("Second turn", { id: "prt_second_user" })], {
+      id: "msg_2000_second_user",
+      created: 1700000005000,
+    })
+    const nextAssistant = assistantMessage([{ id: "prt_second_text", type: "text", text: "Second response" }], {
+      id: "msg_2001_second_assistant",
+      parentID: "msg_2000_second_user",
+      created: 1700000006000,
+    })
+    const timeline = await setupTimeline(page, { messages: [firstUser, aborted, failed, nextUser, nextAssistant] })
+    await timeline.send(status("idle"), 100)
+    const scroller = page.locator(".scroll-view__viewport", { has: page.locator("[data-timeline-row]") })
+    await scroller.evaluate((element) => (element.scrollTop = 0))
+
+    await expect(page.locator('[data-timeline-row="TurnDivider"]')).toHaveCount(1)
+    await expect(page.getByText("Session compacted", { exact: true })).toBeVisible()
+    await expect(page.getByText("Visible provider failure")).toBeVisible()
+    await scroller.evaluate((element) => (element.scrollTop = element.scrollHeight))
+    await expect(page.locator('[data-timeline-row="TurnGap"]')).toBeVisible()
+  })
+
+  test("renders comment strips and historical diff summary overflow", async ({ page }) => {
+    const user = userMessage(
+      [
+        userText("The user made the following comment regarding lines 4 through 8 of src/a.ts: Keep this stable", {
+          id: "prt_comment_only",
+          synthetic: true,
+          metadata: {
+            opencodeComment: {
+              path: "src/a.ts",
+              selection: { startLine: 4, startChar: 0, endLine: 8, endChar: 0 },
+              comment: "Keep this stable",
+            },
+          },
+        }),
+        userText("Continue after the comment", { id: "prt_comment_visible" }),
+      ],
+      { summary: { diffs: Array.from({ length: 11 }, (_, index) => summaryDiff(index)) } },
+    )
+    const nextUser = userMessage(undefined, { id: "msg_2000_diff_next_user", created: 1700000010000 })
+    const nextAssistant = assistantMessage([], {
+      id: "msg_2001_diff_next_assistant",
+      parentID: "msg_2000_diff_next_user",
+      created: 1700000011000,
+    })
+    await setupTimeline(page, { messages: [user, assistantMessage(), nextUser, nextAssistant] })
+    const scroller = page.locator(".scroll-view__viewport", { has: page.locator("[data-timeline-row]") })
+    await scroller.evaluate((element) => (element.scrollTop = 0))
+
+    await expect(page.locator('[data-timeline-row="CommentStrip"]')).toBeVisible()
+    await expect(page.getByText("Keep this stable", { exact: true })).toBeVisible()
+    await expect(page.locator('[data-timeline-row="DiffSummary"]')).toBeVisible()
+    await expect(page.getByText(/show all/i)).toBeVisible()
+  })
+
+  test("renders interruption independently when the turn is not compacted", async ({ page }) => {
+    const user = userMessage()
+    const before = assistantMessage([{ id: "prt_before", type: "text", text: "Before" }], {
+      id: "msg_1001_before",
+      error: { name: "MessageAbortedError", data: { message: "Stopped" } },
+    })
+    const after = assistantMessage([{ id: "prt_after", type: "text", text: "After" }], {
+      id: "msg_1002_after",
+      created: 1700000003000,
+    })
+    await setupTimeline(page, { messages: [user, before, after] })
+
+    await expect(page.getByText("Interrupted", { exact: true })).toBeVisible()
+    const rows = await page
+      .locator('[data-timeline-row="AssistantPart"], [data-timeline-row="TurnDivider"]')
+      .evaluateAll((elements) => elements.map((element) => element.getAttribute("data-timeline-row")))
+    expect(rows).toEqual(["AssistantPart", "TurnDivider", "AssistantPart"])
+  })
+
+  test("renders user image, file attachment, file reference, and agent reference", async ({ page }) => {
+    const text = "Use @explore with @src/a.ts and inspect the attachments"
+    const parts: PartSeed<"user">[] = [
+      userText(text, { id: "prt_user_rich" }),
+      {
+        id: "prt_user_image",
+        type: "file",
+        mime: "image/png",
+        filename: "pixel.png",
+        url: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
+      },
+      {
+        id: "prt_user_attachment",
+        type: "file",
+        mime: "application/json",
+        filename: "tsconfig.json",
+        url: "data:application/json;base64,e30=",
+      },
+      {
+        id: "prt_user_reference",
+        type: "file",
+        mime: "text/plain",
+        filename: "a.ts",
+        url: "src/a.ts",
+        source: { type: "file", path: "src/a.ts", text: { value: "@src/a.ts", start: 18, end: 27 } },
+      },
+      {
+        id: "prt_user_agent",
+        type: "agent",
+        name: "explore",
+        source: { value: "@explore", start: 4, end: 12 },
+      },
+    ]
+    await setupTimeline(page, { messages: [userMessage(parts), assistantMessage()] })
+
+    await expect(page.getByAltText("pixel.png")).toBeVisible()
+    await expect(page.getByText("tsconfig.json")).toBeVisible()
+    await expect(page.getByText("@src/a.ts", { exact: true })).toBeVisible()
+    await expect(page.getByText("@explore", { exact: true })).toBeVisible()
+  })
+})
+
+function editPart(id: string) {
+  return toolPart(
+    id,
+    "edit",
+    "completed",
+    { filePath: "src/a.ts" },
+    {
+      metadata: {
+        filediff: {
+          file: "src/a.ts",
+          additions: 1,
+          deletions: 1,
+          before: "export const value = 1\n",
+          after: "export const value = 2\n",
+        },
+      },
+    },
+  )
+}
+
+function patchPart(id: string) {
+  return toolPart(
+    id,
+    "apply_patch",
+    "completed",
+    { files: ["src/a.ts", "src/b.ts"] },
+    {
+      metadata: {
+        files: [
+          patchFile("src/a.ts", "update"),
+          patchFile("src/b.ts", "add"),
+          patchFile("src/old.ts", "delete"),
+          { ...patchFile("src/moved.ts", "move"), move: "src/new-place.ts" },
+        ],
+      },
+    },
+  )
+}
+
+function patchFile(filePath: string, type: "add" | "update" | "delete" | "move") {
+  return {
+    filePath,
+    relativePath: filePath,
+    type,
+    additions: type === "delete" ? 0 : 1,
+    deletions: type === "add" ? 0 : 1,
+    before: type === "add" ? undefined : "export const before = true\n",
+    after: type === "delete" ? undefined : "export const after = true\n",
+  }
+}
+
+function summaryDiff(index: number) {
+  return {
+    file: `src/diff-${index}.ts`,
+    additions: 1,
+    deletions: 1,
+    patch: `@@ -1 +1 @@\n-export const value = ${index}\n+export const value = ${index + 1}`,
+  }
+}

packages/app/e2e/regression/session-timeline-reasoning-projection.spec.ts

@@ -0,0 +1,93 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  reasoningPart,
+  setupTimeline,
+  status,
+  textPart,
+  toolPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+const profiles = [
+  { name: "summaries off no reasoning", summaries: false, reasoning: "", other: false, thinking: true, body: false },
+  {
+    name: "summaries off reasoning heading",
+    summaries: false,
+    reasoning: "## Inspecting stability",
+    other: false,
+    thinking: true,
+    body: false,
+  },
+  {
+    name: "summaries off with visible tool",
+    summaries: false,
+    reasoning: "## Inspecting stability",
+    other: true,
+    thinking: true,
+    body: false,
+  },
+  { name: "summaries on no content", summaries: true, reasoning: "", other: false, thinking: true, body: false },
+  {
+    name: "summaries on blank reasoning",
+    summaries: true,
+    reasoning: "   ",
+    other: false,
+    thinking: true,
+    body: false,
+  },
+  {
+    name: "summaries on visible reasoning",
+    summaries: true,
+    reasoning: "## Inspecting stability",
+    other: false,
+    thinking: false,
+    body: true,
+  },
+  {
+    name: "summaries on visible tool no reasoning",
+    summaries: true,
+    reasoning: "",
+    other: true,
+    thinking: false,
+    body: false,
+  },
+] as const
+
+for (const profile of profiles) {
+  test(`projects busy reasoning profile ${profile.name}`, async ({ page }) => {
+    const reasoningID = `prt_reasoning_matrix_${profiles.indexOf(profile)}`
+    const parts = [
+      ...(profile.reasoning ? [reasoningPart(reasoningID, profile.reasoning)] : []),
+      ...(profile.other
+        ? [toolPart(`prt_reasoning_tool_${profiles.indexOf(profile)}`, "skill", "running", { name: "inspect" })]
+        : []),
+    ]
+    const timeline = await setupTimeline(page, {
+      messages: [userMessage(), assistantMessage(parts, { completed: false })],
+      settings: { showReasoningSummaries: profile.summaries },
+    })
+    await timeline.send(status("busy"), 150)
+
+    await expect(page.locator('[data-timeline-row="Thinking"]')).toHaveCount(profile.thinking ? 1 : 0)
+    await expect(page.locator(`[data-timeline-part-id="${reasoningID}"]`)).toHaveCount(profile.body ? 1 : 0)
+    if (!profile.summaries && profile.reasoning.trim()) {
+      await expect(page.getByText("Inspecting stability", { exact: true })).toBeVisible()
+    }
+  })
+}
+
+test("does not infer reasoning visibility from provider identity", async ({ page }) => {
+  const timeline = await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage([textPart("prt_provider_text", "No reasoning payload")], { completed: false }),
+    ],
+    settings: { showReasoningSummaries: true },
+  })
+  await timeline.send(status("busy"), 150)
+
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toHaveCount(0)
+  await expect(page.locator('[data-timeline-part-id*="reasoning"]')).toHaveCount(0)
+  await expect(page.locator('[data-timeline-part-id="prt_provider_text"]')).toBeVisible()
+})

packages/app/e2e/regression/session-timeline-reducer-projection.spec.ts

@@ -0,0 +1,43 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  completedAssistantInfo,
+  messageUpdated,
+  partUpdated,
+  setupTimeline,
+  shell,
+  status,
+  textPart,
+  toolPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+test("groups singleton and separated context operations at correct boundaries", async ({ page }) => {
+  const parts = [
+    toolPart("prt_boundary_01_read", "read", "completed", { filePath: "src/a.ts" }),
+    textPart("prt_boundary_02_text", "Boundary text"),
+    toolPart("prt_boundary_03_glob", "glob", "completed", { path: ".", pattern: "**/*.ts" }),
+    toolPart("prt_boundary_04_grep", "grep", "completed", { path: ".", pattern: "stable" }),
+    shell("prt_boundary_05_shell", "completed", "done"),
+    toolPart("prt_boundary_06_list", "list", "completed", { path: "src" }),
+  ]
+  await setupTimeline(page, { messages: [userMessage(), assistantMessage(parts)] })
+
+  await expect(page.locator('[data-timeline-part-ids="prt_boundary_01_read"]')).toBeVisible()
+  await expect(page.locator('[data-timeline-part-ids="prt_boundary_03_glob,prt_boundary_04_grep"]')).toBeVisible()
+  await expect(page.locator('[data-timeline-part-ids="prt_boundary_06_list"]')).toBeVisible()
+  await expect(page.locator('[data-timeline-row="AssistantPart"]')).toHaveCount(5)
+})
+
+test("reducer-hardening: converges when idle arrives before final part and message completion", async ({ page }) => {
+  const textID = "prt_event_order_text"
+  const assistant = assistantMessage([textPart(textID, "Partial")], { completed: false })
+  const timeline = await setupTimeline(page, { messages: [userMessage(), assistant] })
+  await timeline.send(status("busy"), 100)
+  await timeline.send(status("idle"), 100)
+  await timeline.send(partUpdated(textPart(textID, "Final after early idle")), 120)
+  await timeline.send(messageUpdated(completedAssistantInfo(assistant.info)), 250)
+
+  await expect(page.locator('[data-timeline-row="Thinking"]')).toHaveCount(0)
+  await expect(page.locator(`[data-timeline-part-id="${textID}"]`)).toContainText("Final after early idle")
+})

packages/app/e2e/regression/session-timeline-tool-projection.spec.ts

@@ -0,0 +1,99 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  partUpdated,
+  setupTimeline,
+  toolPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+test("renders every tool error outcome without leaking hidden tools", async ({ page }) => {
+  const ordinary = ["bash", "edit", "write", "apply_patch", "webfetch", "websearch", "task", "skill", "mcp_probe"]
+  const parts = ordinary.map((tool, index) =>
+    toolPart(`prt_error_${index}`, tool, "error", errorInput(tool), { error: `${tool} failed visibly` }),
+  )
+  parts.push(
+    toolPart("prt_question_dismissed", "question", "error", questionInput(), {
+      error: "The user dismissed this question",
+    }),
+    toolPart("prt_question_error", "question", "error", questionInput(), { error: "Question transport failed" }),
+    toolPart("prt_todo_error", "todowrite", "error", { todos: [] }, { error: "Hidden todo failure" }),
+  )
+  await setupTimeline(page, { messages: [userMessage(), assistantMessage(parts)] })
+
+  await expect(page.locator('[data-kind="tool-error-card"]')).toHaveCount(ordinary.length + 1)
+  await expect(page.getByText(/dismissed/i)).toBeVisible()
+  await expect(page.locator('[data-timeline-part-id="prt_todo_error"]')).toHaveCount(0)
+  for (let index = 0; index < ordinary.length; index++) {
+    await expect(page.locator(`[data-timeline-part-id="prt_error_${index}"]`)).toBeVisible()
+  }
+})
+
+test("transitions shell and question through running error outcomes", async ({ page }) => {
+  const shellID = "prt_transition_error_shell"
+  const questionID = "prt_transition_error_question"
+  const timeline = await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage(
+        [
+          toolPart(shellID, "bash", "pending", { command: "exit 1" }),
+          toolPart(questionID, "question", "pending", questionInput()),
+        ],
+        { completed: false },
+      ),
+    ],
+  })
+  await timeline.waitForPart(shellID)
+  await expect(page.locator(`[data-timeline-part-id="${questionID}"]`)).toHaveCount(0)
+  await timeline.send(partUpdated(toolPart(shellID, "bash", "running", { command: "exit 1" })), 120)
+  await timeline.send(partUpdated(toolPart(questionID, "question", "running", questionInput())), 180)
+  await expect(page.locator(`[data-timeline-part-id="${questionID}"]`)).toHaveCount(0)
+  await timeline.send(
+    partUpdated(toolPart(shellID, "bash", "error", { command: "exit 1" }, { error: "Command exited 1" })),
+    180,
+  )
+  await timeline.send(
+    partUpdated(
+      toolPart(questionID, "question", "error", questionInput(), { error: "The user dismissed this question" }),
+    ),
+    250,
+  )
+
+  await expect(page.locator(`[data-timeline-part-id="${shellID}"] [data-kind="tool-error-card"]`)).toBeVisible()
+  await expect(page.locator(`[data-timeline-part-id="${questionID}"]`)).toContainText(/dismissed/i)
+})
+
+test("labels all web search provider variants", async ({ page }) => {
+  const parts = [
+    toolPart(
+      "prt_search_parallel",
+      "websearch",
+      "completed",
+      { query: "parallel" },
+      { metadata: { provider: "parallel" } },
+    ),
+    toolPart("prt_search_exa", "websearch", "completed", { query: "exa" }, { metadata: { provider: "exa" } }),
+    toolPart("prt_search_generic", "websearch", "completed", { query: "generic" }),
+  ]
+  await setupTimeline(page, { messages: [userMessage(), assistantMessage(parts)] })
+
+  await expect(page.getByRole("button", { name: /Parallel Web Search/ })).toBeVisible()
+  await expect(page.getByRole("button", { name: /Exa Web Search/ })).toBeVisible()
+  await expect(page.getByRole("button", { name: /^Web Search/ })).toBeVisible()
+})
+
+function questionInput() {
+  return { questions: [{ header: "Stability", question: "Keep it stable?", options: [] }] }
+}
+
+function errorInput(tool: string) {
+  if (tool === "bash") return { command: "exit 1" }
+  if (["edit", "write"].includes(tool)) return { filePath: "src/error.ts", content: "" }
+  if (tool === "apply_patch") return { files: ["src/error.ts"] }
+  if (tool === "webfetch") return { url: "https://example.com" }
+  if (tool === "websearch") return { query: "failure" }
+  if (tool === "task") return { description: "Fail task", subagent_type: "explore" }
+  if (tool === "skill") return { name: "failure" }
+  return { target: "failure" }
+}

packages/app/e2e/regression/session-timeline-tool-state.spec.ts

@@ -0,0 +1,77 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  partUpdated,
+  setupTimeline,
+  toolPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+test("updates expanded web search links without resetting expansion", async ({ page }) => {
+  const searchID = "prt_websearch_mutation"
+  const input = { query: "timeline stability" }
+  const timeline = await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage([toolPart(searchID, "websearch", "completed", input, { output: "https://example.com/one" })]),
+    ],
+  })
+  const wrapper = page.locator(`[data-timeline-part-id="${searchID}"]`)
+  const trigger = wrapper.locator('[data-slot="collapsible-trigger"]')
+  await trigger.click()
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  await timeline.send(
+    partUpdated(
+      toolPart(searchID, "websearch", "completed", input, {
+        output: "https://example.com/one\nhttps://example.com/two",
+      }),
+    ),
+    300,
+  )
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  await expect(wrapper.locator('a[href="https://example.com/two"]')).toBeVisible()
+})
+
+test("preserves an expanded tool error card across duplicate delivery", async ({ page }) => {
+  const toolID = "prt_duplicate_error"
+  const failed = toolPart(toolID, "bash", "error", { command: "exit 1" }, { error: "Command failed visibly" })
+  const timeline = await setupTimeline(page, { messages: [userMessage(), assistantMessage([failed])] })
+  const wrapper = page.locator(`[data-timeline-part-id="${toolID}"]`)
+  const trigger = wrapper.locator('[data-slot="collapsible-trigger"]')
+  await trigger.click()
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  await timeline.send(partUpdated(failed), 150)
+  await timeline.send(partUpdated(failed), 250)
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  await expect(wrapper).toContainText("Command failed visibly")
+})
+
+test("renders multiple question answers and preserves open state on answer updates", async ({ page }) => {
+  const questionID = "prt_multi_question"
+  const input = {
+    questions: [
+      { header: "First", question: "First choice?", options: [] },
+      { header: "Second", question: "Second choice?", options: [], multiple: true },
+    ],
+  }
+  const timeline = await setupTimeline(page, {
+    messages: [
+      userMessage(),
+      assistantMessage([
+        toolPart(questionID, "question", "completed", input, { metadata: { answers: [["A"], ["B", "C"]] } }),
+      ]),
+    ],
+  })
+  const wrapper = page.locator(`[data-timeline-part-id="${questionID}"]`)
+  const trigger = wrapper.locator('[data-slot="collapsible-trigger"]')
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  await timeline.send(
+    partUpdated(
+      toolPart(questionID, "question", "completed", input, { metadata: { answers: [["Updated"], ["B", "C"]] } }),
+    ),
+    300,
+  )
+  await expect(trigger).toHaveAttribute("aria-expanded", "true")
+  await expect(wrapper).toContainText("Updated")
+  await expect(wrapper).toContainText("B, C")
+})

packages/app/e2e/regression/session-timeline-transport.spec.ts

@@ -0,0 +1,116 @@
+import { expect, test } from "@playwright/test"
+import {
+  assistantMessage,
+  partUpdated,
+  setupTimeline,
+  status,
+  textPart,
+  userMessage,
+} from "../performance/timeline-stability/fixture"
+
+test("keeps one connection open while delivering multiple events", async ({ page }) => {
+  const timeline = await setupTimeline(page)
+
+  const first = await timeline.transport.send(partUpdated(textPart("prt_transport_first", "first event")))
+  const second = await timeline.transport.send(partUpdated(textPart("prt_transport_second", "second event")))
+
+  await timeline.waitForPart("prt_transport_first")
+  await timeline.waitForPart("prt_transport_second")
+  expect(first.connectionID).toBe(second.connectionID)
+  expect(await timeline.transport.connections()).toHaveLength(1)
+  expect(await timeline.transport.acknowledgements()).toHaveLength(2)
+})
+
+test("delivers a burst from one stream chunk", async ({ page }) => {
+  const timeline = await setupTimeline(page)
+  const acknowledgements = await timeline.transport.burst([
+    partUpdated(textPart("prt_transport_burst_a", "burst a")),
+    partUpdated(textPart("prt_transport_burst_b", "burst b")),
+  ])
+
+  await timeline.waitForPart("prt_transport_burst_a")
+  await timeline.waitForPart("prt_transport_burst_b")
+  expect(acknowledgements.map((item) => item.chunkCount)).toEqual([1, 1])
+  expect(new Set(acknowledgements.map((item) => item.deliveryID)).size).toBe(2)
+})
+
+test("parses split JSON and a split multibyte code point", async ({ page }) => {
+  const timeline = await setupTimeline(page)
+  const payload = partUpdated(textPart("prt_transport_split", "split snowman \u2603\u2603\u2603"))
+  const encoded = new TextEncoder().encode(`data: ${JSON.stringify(payload)}\n\n`)
+  const snowman = new TextEncoder().encode("\u2603")[0]!
+  const multibyte = encoded.indexOf(snowman)
+
+  const acknowledgement = await timeline.transport.split(payload, [9, multibyte + 1, multibyte + 2])
+
+  await timeline.waitForPart("prt_transport_split")
+  await expect(page.locator('[data-timeline-part-id="prt_transport_split"]')).toContainText(
+    "split snowman \u2603\u2603\u2603",
+  )
+  expect(acknowledgement.chunkCount).toBe(4)
+})
+
+test("delivers server heartbeat without mutating the timeline", async ({ page }) => {
+  const timeline = await setupTimeline(page, {
+    messages: [userMessage(), assistantMessage([textPart("prt_transport_steady", "steady")])],
+  })
+  const before = await page.locator("[data-timeline-row]").allTextContents()
+
+  await timeline.transport.heartbeat()
+  await timeline.settle()
+
+  expect(await page.locator("[data-timeline-row]").allTextContents()).toEqual(before)
+  expect(await timeline.transport.connections()).toHaveLength(1)
+})
+
+test("reconnects after a clean close", async ({ page }) => {
+  const timeline = await setupTimeline(page, { eventRetry: 10 })
+  const first = await timeline.transport.waitForConnection()
+
+  await timeline.transport.close()
+  const second = await timeline.transport.waitForConnection({ after: first.id })
+  await timeline.transport.send(partUpdated(textPart("prt_transport_close", "after close")))
+
+  await timeline.waitForPart("prt_transport_close")
+  expect(second.id).toBeGreaterThan(first.id)
+  expect((await timeline.transport.connections())[0]?.endedBy).toBe("close")
+})
+
+test("reconnects after a stream error", async ({ page }) => {
+  const timeline = await setupTimeline(page, { eventRetry: 10 })
+  const first = await timeline.transport.waitForConnection()
+
+  await timeline.transport.error("contract failure")
+  const second = await timeline.transport.waitForConnection({ after: first.id })
+  await timeline.transport.send(status("busy"))
+
+  await expect.poll(async () => (await timeline.transport.connections()).length).toBe(2)
+  expect(second.id).toBeGreaterThan(first.id)
+  expect((await timeline.transport.connections())[0]?.endedBy).toBe("error")
+})
+
+test("records event IDs and reconnect Last-Event-ID headers", async ({ page }) => {
+  const timeline = await setupTimeline(page, { eventRetry: 10 })
+  const first = await timeline.transport.send(partUpdated(textPart("prt_transport_id", "event with id")), {
+    id: "timeline-event-7",
+  })
+  await timeline.waitForPart("prt_transport_id")
+
+  await timeline.transport.error("retry with event id")
+  const connection = await timeline.transport.waitForConnection({ after: first.connectionID })
+
+  expect(first.eventID).toBe("timeline-event-7")
+  expect(connection.headers["last-event-id"]).toBe("timeline-event-7")
+})
+
+test("passes through non-event fetches", async ({ page }) => {
+  const timeline = await setupTimeline(page)
+
+  const health = await page.evaluate(async () => {
+    const response = await fetch("/global/health")
+    return response.json()
+  })
+
+  expect(health).toEqual({ healthy: true })
+  expect(await timeline.transport.connections()).toHaveLength(1)
+})