diff --git a/tests/e2e/perf.e2e.ts b/tests/e2e/perf.e2e.ts
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+/**
+ * Perf lane: a MEASUREMENT harness, not a pass/fail gate (beyond the mount
+ * sanity it shares with the other lanes).
+ *
+ * It exists to put NUMBERS on the sidebar's client-side cost in a real
+ * mounted `dsh web` (the same scratch-profile boot as scripts/e2e-mount.sh):
+ *
+ *   1. mount latency  — navigationStart → `[data-dsh-better-sidebar]`
+ *      attached, sampled by an init-script probe (rAF loop, so it catches
+ *      the attach even before Playwright's first poll);
+ *   2. longtasks      — every >50ms main-thread task from first paint
+ *      through the full built-in tab sweep (the heaviest mount surface);
+ *   3. bundle cost    — per-resource transferSize/decodedBodySize/duration
+ *      for everything sidebar-shaped the page fetched;
+ *   4. bottom-drag frames — rAF frame-interval p95/max while dragging the
+ *      bottom strip (the layout-push hot path), plus the width-leak guard
+ *      from the drag lane (a closed right panel must push 0).
+ *
+ * Results print as single-line `PERF_JSON` records (plus a `PERF_SUMMARY`
+ * aggregate) so a shell run can `tee` the log and grep them out; the
+ * before/after comparison lives in docs/plans/2026-08-31-perf-optimization.md.
+ * Run it 3× and take the median of each metric — single-run numbers on a
+ * developer machine are noise, not baselines.
+ */
+import { mkdirSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { test, expect, type APIRequestContext, type Page } from '@playwright/test'
+import { PAGE_URL, createHostApi, hostRpc } from './host'
+
+/** This lane's own workspace (lanes run serially against one server — never
+ *  share seed paths with mount/drag). */
+const WORKSPACE_PATH = process.env.DSH_E2E_PERF_WORKSPACE ?? join(tmpdir(), 'dsh-e2e-perf-workspace')
+
+/** Built-in tab titles the sweep drives (en-US copy; follows DSH locale). */
+const BUILTIN_TABS = ['Files', 'Changes', 'Tasks', 'Side Chat (beta)', 'Terminal', 'Browser']
+
+let api: APIRequestContext
+
+async function seedSession(): Promise<void> {
+  mkdirSync(WORKSPACE_PATH, { recursive: true })
+  writeFileSync(join(WORKSPACE_PATH, 'seed.txt'), 'perf lane\n')
+  const workspace = await hostRpc<{ workspace: { workspaceId: string } }>(api, 'workspace.create', { path: WORKSPACE_PATH })
+  await hostRpc(api, 'session.create', { workspaceId: workspace.value.workspace.workspaceId })
+}
+
+test.beforeAll(async () => {
+  api = await createHostApi()
+  await seedSession()
+})
+
+test.afterAll(async () => {
+  await api?.dispose()
+})
+
+/** The longtask collector + mount-attach probe, injected before ANY page
+ *  script runs (longtasks observed late are gone forever). */
+const PERF_PROBE = `(() => {
+  const perf = { mountAt: null, longtasks: [] }
+  window.__perfProbe = perf
+  new PerformanceObserver(list => {
+    for (const entry of list.getEntries()) perf.longtasks.push({ start: entry.startTime, duration: entry.duration })
+  }).observe({ entryTypes: ['longtask'] })
+  const check = () => {
+    if (document.querySelector('[data-dsh-better-sidebar]') !== null) { perf.mountAt = performance.now(); return }
+    requestAnimationFrame(check)
+  }
+  requestAnimationFrame(check)
+})()`
+
+function percentile(values: number[], p: number): number {
+  if (values.length === 0) return 0
+  const sorted = [...values].sort((a, b) => a - b)
+  const index = Math.min(sorted.length - 1, Math.max(0, Math.ceil((p / 100) * sorted.length) - 1))
+  return sorted[index]!
+}
+
+interface ResourceStat {
+  name: string
+  transferSize: number
+  decodedBodySize: number
+  duration: number
+}
+
+/** Dismiss whatever onboarding takeover is present (same dance as the other
+ *  lanes) so the pointer can reach the sidebar's controls. */
+async function dismissOnboarding(page: Page): Promise<void> {
+  try {
+    await expect
+      .poll(() => page.getByRole('button', { name: /^(Continue|Configure later)$/ }).count(), { timeout: 60_000 })
+      .toBeGreaterThan(0)
+  } catch {
+    console.warn('[e2e-perf] no onboarding takeover appeared; proceeding')
+  }
+  for (let round = 0; round < 8; round++) {
+    let dismissed = false
+    for (const name of ['Continue', 'Configure later']) {
+      const button = page.getByRole('button', { name, exact: true }).first()
+      if ((await button.count()) === 0) continue
+      try {
+        await button.click({ timeout: 4_000 })
+        dismissed = true
+        await page.waitForTimeout(1_000)
+      } catch {
+        // Masked by a takeover stacked above; retry in the next round.
+      }
+    }
+    if (!dismissed) break
+  }
+}
+
+test('measure: mount latency, longtasks and bundle cost through a full tab sweep', async ({ page }) => {
+  await page.addInitScript(PERF_PROBE)
+  await page.goto(PAGE_URL, { waitUntil: 'domcontentloaded' })
+  await expect(page.locator('#root > *')).not.toHaveCount(0, { timeout: 90_000 })
+  const sidebar = page.locator('[data-dsh-better-sidebar]')
+  await expect(sidebar).toBeAttached({ timeout: 90_000 })
+
+  const mountLatency = await page.evaluate(() =>
+    (window as unknown as { __perfProbe: { mountAt: number | null } }).__perfProbe.mountAt)
+  expect(mountLatency, 'the mount probe must have caught the attach').not.toBeNull()
+
+  await dismissOnboarding(page)
+
+  // Open the panel, then sweep every built-in tab through the "+" menu — the
+  // same crash-sweep surface as mount.e2e.ts, reused here as the workload.
+  const expandButton = sidebar.getByRole('button', { name: 'Expand sidebar' })
+  await expect(expandButton).toHaveCount(1)
+  await expandButton.click()
+  await expect
+    .poll(async () => {
+      const value = await page.evaluate(() => document.documentElement.style.getPropertyValue('--dsh-sidebar-width'))
+      return value !== '' && value !== '0px'
+    }, { timeout: 90_000 })
+    .toBe(true)
+
+  const newTabButton = sidebar.getByRole('button', { name: 'New tab' }).first()
+  for (const title of BUILTIN_TABS) {
+    await newTabButton.click()
+    const item = page.getByRole('menuitem', { name: title }).first()
+    await expect(item, `built-in tab "${title}" must be offered by the + menu`).toHaveCount(1)
+    await item.click()
+    await page.waitForTimeout(1_500)
+  }
+
+  // Let the last tab settle (lazy chunks + first render) before reading.
+  await page.waitForTimeout(2_000)
+  const probe = await page.evaluate(() => (window as unknown as { __perfProbe: { mountAt: number | null; longtasks: Array<{ start: number; duration: number }> } }).__perfProbe)
+  const resources = await page.evaluate(() =>
+    performance.getEntriesByType('resource')
+      .filter(entry => /sidebar/.test(entry.name))
+      .map(entry => ({
+        name: entry.name.replace(/^https?:\/\/[^/]+/, ''),
+        transferSize: entry.transferSize,
+        decodedBodySize: entry.decodedBodySize,
+        duration: entry.duration,
+      })) as ResourceStat[],
+  )
+
+  const longtasks = probe.longtasks ?? []
+  const record = {
+    metric: 'mount-sweep',
+    mountLatencyMs: mountLatency,
+    longtaskCount: longtasks.length,
+    longtaskTotalMs: Math.round(longtasks.reduce((sum, task) => sum + task.duration, 0)),
+    longtaskMaxMs: longtasks.reduce((max, task) => Math.max(max, task.duration), 0),
+    resourceCount: resources.length,
+    resourceTransferTotal: resources.reduce((sum, r) => sum + r.transferSize, 0),
+    resourceDecodedTotal: resources.reduce((sum, r) => sum + r.decodedBodySize, 0),
+    resources,
+  }
+  console.log(`PERF_JSON ${JSON.stringify(record)}`)
+  console.log(
+    `PERF_SUMMARY mount=${Math.round(mountLatency!)}ms longtasks=${record.longtaskCount}/${record.longtaskTotalMs}ms `
+    + `resources=${record.resourceCount}/${(record.resourceTransferTotal / 1024).toFixed(0)}KiB`,
+  )
+})
+
+test('measure: bottom-strip drag frame pacing', async ({ page }) => {
+  await page.addInitScript(PERF_PROBE)
+  await page.goto(PAGE_URL, { waitUntil: 'domcontentloaded' })
+  await expect(page.locator('#root > *')).not.toHaveCount(0, { timeout: 90_000 })
+  const sidebar = page.locator('[data-dsh-better-sidebar]')
+  await expect(sidebar).toBeAttached({ timeout: 90_000 })
+  await dismissOnboarding(page)
+
+  // The right panel stays CLOSED on purpose: the bottom drag must not push
+  // the host layout in that pose (the width-leak regression the drag lane
+  // locks; recorded here as a number-adjacent guard for the perf story).
+  const bottomExpand = sidebar.getByRole('button', { name: 'Expand bottom panel' })
+  await expect(bottomExpand).toHaveCount(1)
+  await bottomExpand.click()
+  await expect
+    .poll(async () => {
+      const value = await page.evaluate(() => parseFloat(document.documentElement.style.getPropertyValue('--dsh-sidebar-height')) || 0)
+      return value
+    }, { timeout: 90_000 })
+    .toBeGreaterThan(0)
+
+  // Frame-pacing sampler: rAF timestamps during the drag only.
+  await page.evaluate(() => {
+    const frames: number[] = []
+    const pushWidths: number[] = []
+    const loop = (): void => {
+      frames.push(performance.now())
+      const width = parseFloat(document.documentElement.style.getPropertyValue('--dsh-sidebar-width'))
+      pushWidths.push(Number.isNaN(width) ? 0 : width)
+      requestAnimationFrame(loop)
+    }
+    requestAnimationFrame(loop)
+    ;(window as unknown as { __dragFrames: number[]; __dragPushWidths: number[] }).__dragFrames = frames
+    ;(window as unknown as { __dragPushWidths: number[] }).__dragPushWidths = pushWidths
+  })
+
+  // The panel slides down into place on expand: read the strip only after it
+  // settles at innerHeight - the pushed height (a mid-animation box makes the
+  // pointerdown land off-strip and the drag never starts).
+  await expect
+    .poll(async () => {
+      const probe = await page.evaluate(() => {
+        const bottom = document.querySelector('[data-dsh-better-sidebar] [data-dsh-bottom-panel]')
+        if (bottom === null) return null
+        const strip = [...bottom.querySelectorAll<HTMLElement>('*')].find(el => getComputedStyle(el).cursor === 'row-resize')
+        if (strip === undefined) return null
+        const r = strip.getBoundingClientRect()
+        const heightVar = parseFloat(document.documentElement.style.getPropertyValue('--dsh-sidebar-height')) || 0
+        return Math.abs((r.y + r.height / 2) - (window.innerHeight - heightVar))
+      })
+      return probe === null ? Number.NaN : probe
+    }, { timeout: 30_000 })
+    .toBeLessThanOrEqual(8)
+  const stripPoint = await page.evaluate(() => {
+    const bottom = document.querySelector('[data-dsh-better-sidebar] [data-dsh-bottom-panel]')
+    if (bottom === null) return null
+    const strip = [...bottom.querySelectorAll<HTMLElement>('*')].find(el => getComputedStyle(el).cursor === 'row-resize')
+    if (strip === undefined) return null
+    const r = strip.getBoundingClientRect()
+    return { x: r.x + r.width / 2, y: r.y + r.height / 2 }
+  })
+  expect(stripPoint, 'the bottom drag strip must be present').not.toBeNull()
+
+  const t0 = await page.evaluate(() => performance.now())
+  await page.mouse.move(stripPoint!.x, stripPoint!.y)
+  await page.mouse.down()
+  for (let i = 1; i <= 12; i++) {
+    await page.mouse.move(stripPoint!.x, stripPoint!.y - i * 8, { steps: 2 })
+    await page.waitForTimeout(40)
+  }
+  await page.mouse.up()
+  const t1 = await page.evaluate(() => performance.now())
+  await page.waitForTimeout(300)
+
+  const { frames, widths } = await page.evaluate(() => ({
+    frames: (window as unknown as { __dragFrames: number[] }).__dragFrames,
+    widths: (window as unknown as { __dragPushWidths: number[] }).__dragPushWidths,
+  }))
+  const dragFrames = frames.filter(t => t >= t0 && t <= t1)
+  const intervals: number[] = []
+  for (let i = 1; i < dragFrames.length; i++) intervals.push(dragFrames[i]! - dragFrames[i - 1]!)
+  const dragWidths = widths.slice(Math.max(0, frames.indexOf(dragFrames[0] ?? frames[0]!)))
+  const record = {
+    metric: 'bottom-drag',
+    frames: dragFrames.length,
+    intervalMedianMs: Math.round(percentile(intervals, 50)),
+    intervalP95Ms: Math.round(percentile(intervals, 95)),
+    intervalMaxMs: Math.round(Math.max(0, ...intervals)),
+    pushWidthLeakMax: Math.max(0, ...dragWidths),
+  }
+  console.log(`PERF_JSON ${JSON.stringify(record)}`)
+  console.log(
+    `PERF_SUMMARY drag-frames=${record.frames} p50=${record.intervalMedianMs}ms p95=${record.intervalP95Ms}ms `
+    + `max=${record.intervalMaxMs}ms widthLeak=${record.pushWidthLeakMax}px`,
+  )
+  expect(record.pushWidthLeakMax, 'the closed right panel must push 0 width during the bottom drag').toBe(0)
+})