diff --git a/desktop/playwright.perf-webkit.config.ts b/desktop/playwright.perf-webkit.config.ts
new file mode 100644
index 0000000000..1a79732f17
--- /dev/null
+++ b/desktop/playwright.perf-webkit.config.ts
@@ -0,0 +1,23 @@
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./tests/e2e",
+  timeout: 900_000,
+  retries: 0,
+  workers: 1,
+  reporter: [["list"]],
+  use: { baseURL: "http://127.0.0.1:4173" },
+  projects: [
+    {
+      name: "perf-webkit",
+      testMatch: ["**/*.perf.ts"],
+      use: { ...devices["Desktop Safari"] },
+    },
+  ],
+  webServer: {
+    command: "python3 -m http.server 4173 -d dist",
+    cwd: ".",
+    reuseExistingServer: true,
+    url: "http://127.0.0.1:4173",
+  },
+});
diff --git a/desktop/tests/e2e/upscroll-1px-live.perf.ts b/desktop/tests/e2e/upscroll-1px-live.perf.ts
new file mode 100644
index 0000000000..897f1c8e92
--- /dev/null
+++ b/desktop/tests/e2e/upscroll-1px-live.perf.ts
@@ -0,0 +1,433 @@
+import { expect, test } from "@playwright/test";
+import { decode } from "nostr-tools/nip19";
+import { getPublicKey } from "nostr-tools/pure";
+
+import { installRelayBridge } from "../helpers/bridge";
+
+/**
+ * LIVE 1-PIXEL UPSCROLL PROBE — Tyler's ask (2026-07-08):
+ * run the GUI against the LIVE staging relay, scroll up 1px at a time for
+ * many thousands of pixels (crossing multiple real fetchOlder scrollback
+ * loads in #buzz-bugs), and record how many pixels of VISIBLE MOVEMENT occur
+ * per 1px of input.
+ *
+ * Method: same live-relay harness as scrollback-buzzbugs.perf.ts (identity
+ * override, port-forward + Host rewrite, ACAO injection). Actuation is a
+ * synchronous `scrollTop -= 1` + scroll event per step with one RAF settle,
+ * batched in-page (800 steps/evaluate) so 15k+ steps are feasible. Per step
+ * we record:
+ *   move       = tracked center row's rect.top delta   (the felt movement)
+ *   appliedTop = scrollTop_before - scrollTop_after    (what the scroller did)
+ *   mounted    = [data-message-id] count               (prepend detection)
+ *   fetch      = __CHANNEL_WINDOW_FETCH_COUNT__        (fetchOlder activity)
+ * Smooth = move === +1 every step. Jank = |move| >> 1 (realization lurch,
+ * anchor jump, prepend re-anchor).
+ *
+ * WKWebView mirror: `overflow-anchor: none` forced on the scroller (the
+ * shipped engine has no scroll anchoring); the production computed value is
+ * logged first, un-forced, for the anchor-contract record.
+ *
+ * Run:
+ *   kubectl --context bke-coder-stage -n sprout port-forward svc/sprout-relay 13000:3000 &
+ *   BUZZ_E2E_RELAY_URL=http://127.0.0.1:13000 \
+ *   BUZZ_COMMUNITY_HOST=sprout-oss.stage.blox.sqprod.co \
+ *   BUZZ_PERF_NSEC=nsec1... \
+ *   npx playwright test --config=playwright.perf.config.ts upscroll-1px-live
+ */
+
+const RELAY_HTTP =
+  process.env.BUZZ_E2E_RELAY_URL ?? "https://sprout-oss.stage.blox.sqprod.co";
+const NSEC = process.env.BUZZ_PERF_NSEC ?? "";
+const COMMUNITY_HOST = process.env.BUZZ_COMMUNITY_HOST ?? "";
+const TARGET_CHANNEL = process.env.BUZZ_PERF_CHANNEL ?? "buzz-bugs";
+// Stop when BOTH: at least this many fetchOlder pages landed AND this many
+// pixels walked — or the step cap / top of history, whichever first.
+const MIN_FETCHES = Number(process.env.BUZZ_PERF_MIN_FETCHES ?? 3);
+const MIN_PX = Number(process.env.BUZZ_PERF_MIN_PX ?? 8000);
+const MAX_STEPS = Number(process.env.BUZZ_PERF_MAX_STEPS ?? 20000);
+const BATCH = 800;
+const SAFE_MARGIN = 60;
+// Pixels per input step (1 = original probe; 100+ = Tyler's big-notch repro).
+const STEP_PX = Number(process.env.BUZZ_PERF_STEP_PX ?? 1);
+
+const IDENTITY_OVERRIDE_KEY = "buzz:e2e-identity-override.v1";
+const ONBOARDING_PREFIX = "buzz-onboarding-complete.v1:";
+const WELCOME_PREFIX = "buzz-welcome-channel-ensured.v2:";
+const REAL_CHROME_UA =
+  "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36";
+
+test.use({ userAgent: REAL_CHROME_UA });
+
+function deriveIdentity(nsec: string) {
+  const decoded = decode(nsec.trim());
+  if (decoded.type !== "nsec") throw new Error("BUZZ_PERF_NSEC is not an nsec");
+  const skBytes = decoded.data as Uint8Array;
+  return {
+    privateKey: Buffer.from(skBytes).toString("hex"),
+    pubkey: getPublicKey(skBytes),
+    username: "perf-eva",
+  };
+}
+
+type Step = {
+  i: number;
+  move: number | null; // tracked row rect.top delta; null = row lost this step
+  appliedTop: number;
+  mounted: number;
+  fetch: number;
+  scrollTop: number;
+  repick: boolean; // tracked row re-picked before this step
+};
+
+test("MEASURE: live 1px upscroll movement profile", async ({ page }) => {
+  test.setTimeout(900_000);
+  if (!NSEC) throw new Error("Set BUZZ_PERF_NSEC to a real member nsec");
+  const identity = deriveIdentity(NSEC);
+
+  await installRelayBridge(page, "tyler");
+  const wsUrl = RELAY_HTTP.replace(/^http/, "ws");
+  await page.addInitScript(
+    ({ ident, onboardingPrefix, welcomePrefix, relayUrl, overrideKey }) => {
+      window.localStorage.setItem(overrideKey, JSON.stringify(ident));
+      window.localStorage.setItem(`${onboardingPrefix}${ident.pubkey}`, "true");
+      window.localStorage.setItem(
+        `${welcomePrefix}${encodeURIComponent(relayUrl)}:${ident.pubkey}`,
+        "true",
+      );
+      const w = window as unknown as { __BUZZ_E2E__?: Record<string, unknown> };
+      w.__BUZZ_E2E__ = { ...(w.__BUZZ_E2E__ ?? {}), identity: ident };
+    },
+    {
+      ident: identity,
+      onboardingPrefix: ONBOARDING_PREFIX,
+      welcomePrefix: WELCOME_PREFIX,
+      relayUrl: wsUrl,
+      overrideKey: IDENTITY_OVERRIDE_KEY,
+    },
+  );
+
+  const relayHost = new URL(RELAY_HTTP).host;
+  await page.route(
+    (url) => url.host === relayHost,
+    async (route) => {
+      const req = route.request();
+      const fwd = { ...req.headers(), "user-agent": REAL_CHROME_UA };
+      delete fwd["sec-ch-ua"];
+      delete fwd["sec-ch-ua-mobile"];
+      delete fwd["sec-ch-ua-platform"];
+      if (COMMUNITY_HOST) fwd.host = COMMUNITY_HOST;
+      // The port-forward can drop mid-run; retry a few times rather than
+      // aborting a 15k-step walk on one transient ECONNREFUSED.
+      let resp: Awaited<ReturnType<typeof route.fetch>> | null = null;
+      for (let attempt = 0; attempt < 5; attempt++) {
+        try {
+          resp = await route.fetch({ headers: fwd });
+          break;
+        } catch (err) {
+          if (attempt === 4) throw err;
+          await new Promise((r) => setTimeout(r, 1500));
+        }
+      }
+      if (!resp) throw new Error("unreachable");
+      const body = req.postData() ?? "";
+      if (body.includes("before_id") || body.includes("until")) {
+        console.log(
+          `[net] ${new URL(req.url()).pathname} ${resp.status()} ${(await resp.body()).byteLength}b body=${body.slice(0, 140)}`,
+        );
+      }
+      const headers = { ...resp.headers() };
+      headers["access-control-allow-origin"] = "*";
+      headers["access-control-allow-headers"] = "*";
+      headers["access-control-allow-methods"] = "*";
+      await route.fulfill({ response: resp, headers, body: await resp.body() });
+    },
+  );
+
+  await page.goto("/");
+  await page.getByTestId("app-sidebar").waitFor({ state: "visible" });
+  const chan = page.getByTestId(`channel-${TARGET_CHANNEL}`).first();
+  await chan.waitFor({ state: "visible", timeout: 45_000 });
+  await chan.click();
+  await page
+    .locator('[data-testid="message-timeline"] [data-message-id]')
+    .first()
+    .waitFor({ state: "visible", timeout: 30_000 });
+  await page.waitForTimeout(3000); // let the open burst settle
+
+  const timeline = page.getByTestId("message-timeline");
+
+  // Production anchor contract, read BEFORE forcing anything.
+  const anchor = await timeline.evaluate((el) => ({
+    supports: CSS.supports("overflow-anchor", "none"),
+    computed: getComputedStyle(el as HTMLElement).overflowAnchor ?? "(none)",
+  }));
+  console.log(
+    `[anchor] CSS.supports=${anchor.supports} production computed=${anchor.computed}`,
+  );
+  // WKWebView mirror: no engine-side anchoring during the walk.
+  await timeline.evaluate((el) => {
+    (el as HTMLElement).style.overflowAnchor = "none";
+  });
+
+  // Pin to true bottom.
+  await timeline.evaluate((element) => {
+    const el = element as HTMLDivElement;
+    el.scrollTop = el.scrollHeight;
+    el.dispatchEvent(new Event("scroll", { bubbles: true }));
+  });
+  await page.waitForTimeout(500);
+
+  const all: Step[] = [];
+  const prependCommits: Array<{
+    step: number;
+    mountedFrom: number;
+    mountedTo: number;
+    scrollTopTo: number;
+    feltJump: number | null;
+  }> = [];
+  let ended = "cap";
+  let stepIndex = 0;
+
+  while (stepIndex < MAX_STEPS) {
+    const batch = await timeline.evaluate(
+      async (
+        element,
+        args: { n: number; margin: number; base: number; step: number },
+      ) => {
+        const el = element as HTMLDivElement;
+        const probe = window as unknown as {
+          __CHANNEL_WINDOW_FETCH_COUNT__?: number;
+        };
+        const out: Array<{
+          i: number;
+          move: number | null;
+          appliedTop: number;
+          mounted: number;
+          fetch: number;
+          scrollTop: number;
+          repick: boolean;
+        }> = [];
+        let end = "";
+        let trackedId: string | null = null;
+
+        const pick = (): { id: string; top: number } | null => {
+          const box = el.getBoundingClientRect();
+          const mid = box.top + box.height / 2;
+          let best: { id: string; top: number; d: number } | null = null;
+          for (const row of el.querySelectorAll<HTMLElement>(
+            "[data-message-id]",
+          )) {
+            const r = row.getBoundingClientRect();
+            if (
+              r.top <= box.top + args.margin ||
+              r.bottom >= box.bottom - args.margin
+            )
+              continue;
+            const d = Math.abs((r.top + r.bottom) / 2 - mid);
+            if (!best || d < best.d)
+              best = { id: row.dataset.messageId ?? "", top: r.top, d };
+          }
+          return best && best.id ? { id: best.id, top: best.top } : null;
+        };
+
+        const locate = (id: string): number | null => {
+          const box = el.getBoundingClientRect();
+          const row = el.querySelector<HTMLElement>(
+            `[data-message-id="${CSS.escape(id)}"]`,
+          );
+          if (!row) return null;
+          const r = row.getBoundingClientRect();
+          if (
+            r.top <= box.top + args.margin ||
+            r.bottom >= box.bottom - args.margin
+          )
+            return null; // left safe band
+          return r.top;
+        };
+
+        for (let i = 0; i < args.n; i++) {
+          if (el.scrollTop <= 0) {
+            end = "top";
+            break;
+          }
+          let repick = false;
+          let beforeTop = trackedId ? locate(trackedId) : null;
+          if (beforeTop === null) {
+            const p = pick();
+            repick = true;
+            if (!p) {
+              trackedId = null;
+            } else {
+              trackedId = p.id;
+              beforeTop = p.top;
+            }
+          }
+          const beforeScroll = el.scrollTop;
+
+          el.scrollTop = Math.max(0, el.scrollTop - args.step);
+          el.dispatchEvent(new Event("scroll", { bubbles: true }));
+          await new Promise<void>((r) => requestAnimationFrame(() => r()));
+
+          const afterTop = trackedId ? locate(trackedId) : null;
+          out.push({
+            i: args.base + i,
+            move:
+              beforeTop !== null && afterTop !== null
+                ? afterTop - beforeTop
+                : null,
+            appliedTop: beforeScroll - el.scrollTop,
+            mounted: el.querySelectorAll("[data-message-id]").length,
+            fetch: probe.__CHANNEL_WINDOW_FETCH_COUNT__ ?? 0,
+            scrollTop: el.scrollTop,
+            repick,
+          });
+        }
+        return { out, end };
+      },
+      { n: BATCH, margin: SAFE_MARGIN, base: stepIndex, step: STEP_PX },
+    );
+
+    all.push(...batch.out);
+    stepIndex += batch.out.length;
+    const fetches = all.length ? all[all.length - 1].fetch : 0;
+    const walked = all.reduce((s, x) => s + Math.max(0, x.appliedTop), 0);
+    console.log(
+      `[progress] steps=${stepIndex} walkedPx=${walked.toFixed(0)} fetches=${fetches} mounted=${all[all.length - 1]?.mounted ?? "?"} scrollTop=${all[all.length - 1]?.scrollTop.toFixed(0) ?? "?"}`,
+    );
+    if (batch.end === "top") {
+      // We are at scrollTop=0. A fetchOlder page may be in flight (the 600px
+      // sentinel fires well before the top). Wait for the prepend to commit,
+      // measure the FELT jump it causes on a visible row, then keep walking
+      // into the paged-in history. Only a timeout means true top-of-history.
+      const beforeWait = await timeline.evaluate((element) => {
+        const el = element as HTMLDivElement;
+        const first = el.querySelector<HTMLElement>("[data-message-id]");
+        return {
+          mounted: el.querySelectorAll("[data-message-id]").length,
+          scrollTop: el.scrollTop,
+          firstId: first?.dataset.messageId ?? null,
+          firstTop: first?.getBoundingClientRect().top ?? 0,
+        };
+      });
+      const grew = await page
+        .waitForFunction(
+          (n) =>
+            document.querySelectorAll(
+              '[data-testid="message-timeline"] [data-message-id]',
+            ).length > n,
+          beforeWait.mounted,
+          { timeout: 15_000 },
+        )
+        .then(() => true)
+        .catch(() => false);
+      if (!grew) {
+        ended = "top-of-history";
+        break;
+      }
+      await page.waitForTimeout(300); // let the re-anchor settle
+      const afterWait = await timeline.evaluate(
+        (element, firstId: string | null) => {
+          const el = element as HTMLDivElement;
+          const row = firstId
+            ? el.querySelector<HTMLElement>(
+                `[data-message-id="${CSS.escape(firstId)}"]`,
+              )
+            : null;
+          return {
+            mounted: el.querySelectorAll("[data-message-id]").length,
+            scrollTop: el.scrollTop,
+            firstTop: row?.getBoundingClientRect().top ?? null,
+          };
+        },
+        beforeWait.firstId,
+      );
+      const felt =
+        afterWait.firstTop !== null
+          ? afterWait.firstTop - beforeWait.firstTop
+          : null;
+      console.log(
+        `[prepend-commit] mounted ${beforeWait.mounted}->${afterWait.mounted} scrollTop ${beforeWait.scrollTop.toFixed(0)}->${afterWait.scrollTop.toFixed(0)} FELT-JUMP of anchored row=${felt === null ? "row-gone" : `${felt.toFixed(1)}px`}`,
+      );
+      prependCommits.push({
+        step: stepIndex,
+        mountedFrom: beforeWait.mounted,
+        mountedTo: afterWait.mounted,
+        scrollTopTo: afterWait.scrollTop,
+        feltJump: felt,
+      });
+      continue;
+    }
+    if (fetches >= MIN_FETCHES && walked >= MIN_PX) {
+      ended = "target-reached";
+      break;
+    }
+  }
+
+  // ---- REPORT ----
+  const scored = all.filter((s) => s.move !== null) as Array<
+    Step & { move: number }
+  >;
+  const hist = new Map<number, number>();
+  for (const s of scored) {
+    const b = Math.round(s.move);
+    hist.set(b, (hist.get(b) ?? 0) + 1);
+  }
+  console.log(
+    `\n=== LIVE UPSCROLL PROFILE (step=${STEP_PX}px): #${TARGET_CHANNEL} ===`,
+  );
+  console.log(
+    `relay=${RELAY_HTTP} steps=${all.length} scored=${scored.length} ended=${ended}`,
+  );
+  console.log(
+    `total input px=${all.length} · applied px=${all.reduce((s, x) => s + Math.max(0, x.appliedTop), 0)} · fetchOlder pages=${all.length ? all[all.length - 1].fetch : 0}`,
+  );
+  console.log("\n--- movement histogram (px moved per 1px input, rounded) ---");
+  for (const [k, v] of [...hist.entries()].sort((a, b) => a[0] - b[0]))
+    console.log(`  ${String(k).padStart(6)}px : ${v}`);
+
+  const smooth = scored.filter(
+    (s) => Math.abs(s.move - STEP_PX) <= Math.max(0.5, STEP_PX * 0.1),
+  ).length;
+  console.log(
+    `\nsmooth steps (move≈STEP=${STEP_PX}px): ${smooth}/${scored.length} = ${((100 * smooth) / Math.max(1, scored.length)).toFixed(2)}%`,
+  );
+
+  const jumps = scored
+    .filter((s) => Math.abs(s.move - STEP_PX) > Math.max(2, STEP_PX * 0.25))
+    .sort((a, b) => Math.abs(b.move) - Math.abs(a.move));
+  console.log(
+    `\n--- jumps |move-STEP| > max(2, STEP*0.25): ${jumps.length} ---`,
+  );
+  let prevFetch = 0;
+  const fetchSteps: number[] = [];
+  for (const s of all) {
+    if (s.fetch > prevFetch) fetchSteps.push(s.i);
+    prevFetch = s.fetch;
+  }
+  console.log(`fetchOlder landed at steps: ${fetchSteps.join(", ")}`);
+  for (const s of jumps.slice(0, 40))
+    console.log(
+      `  step=${s.i} move=${s.move.toFixed(1)}px applied=${s.appliedTop.toFixed(1)} mounted=${s.mounted} fetch=${s.fetch} scrollTop=${s.scrollTop.toFixed(0)} repick=${s.repick}`,
+    );
+
+  console.log(
+    `\n--- prepend commits observed at top (${prependCommits.length}) ---`,
+  );
+  for (const c of prependCommits)
+    console.log(
+      `  step=${c.step} mounted ${c.mountedFrom}->${c.mountedTo} scrollTopAfter=${c.scrollTopTo.toFixed(0)} feltJump=${c.feltJump === null ? "row-gone" : `${c.feltJump.toFixed(1)}px`}`,
+    );
+
+  // Prepend re-anchors (scrollTop jumped up) for the record.
+  const reanchors = all.filter((s) => s.appliedTop < -50);
+  console.log(
+    `\n--- prepend re-anchors (appliedTop < -50): ${reanchors.length} ---`,
+  );
+  for (const s of reanchors.slice(0, 20))
+    console.log(
+      `  step=${s.i} appliedTop=${s.appliedTop.toFixed(0)} mounted=${s.mounted} fetch=${s.fetch} scrollTop=${s.scrollTop.toFixed(0)}`,
+    );
+  console.log("==============================================\n");
+
+  expect(all.length * STEP_PX).toBeGreaterThan(1000);
+});
diff --git a/desktop/tests/e2e/upscroll-trackpad-live.perf.ts b/desktop/tests/e2e/upscroll-trackpad-live.perf.ts
new file mode 100644
index 0000000000..e38d2444b7
--- /dev/null
+++ b/desktop/tests/e2e/upscroll-trackpad-live.perf.ts
@@ -0,0 +1,358 @@
+import { expect, test } from "@playwright/test";
+import { decode } from "nostr-tools/nip19";
+import { getPublicKey } from "nostr-tools/pure";
+
+import { installRelayBridge } from "../helpers/bridge";
+
+/**
+ * LIVE TRACKPAD-MOMENTUM UPSCROLL PROBE — Tyler: "the jumping is bad even on
+ * PR 1650" with a trackpad. The prior probes settled 1-2 RAFs after every
+ * step; a trackpad does not. This probe replicates macOS trackpad swipes:
+ * trusted CDP mouseWheel events at ~8ms spacing — finger ramp then an
+ * exponentially-decaying momentum tail — fired CONTINUOUSLY through real
+ * fetchOlder commits in #buzz-bugs against live staging. No settling.
+ *
+ * Measurement is a per-RAF in-page sampler, independent of input timing:
+ * every frame it records scrollTop + the tracked center row's rect.top +
+ * mounted count + fetch count. For a solid page,
+ *   rowMove(frame) = rect.top delta = scrollTop_before - scrollTop_after.
+ * Per-frame deviation = rowMove - appliedScroll. Nonzero = content shifted
+ * under the viewport that the input didn't ask for — the felt jump —
+ * regardless of whether it came from CV realization, a losing compensation
+ * race, or a prepend anchor miss.
+ *
+ * Run: same env as upscroll-1px-live.perf.ts.
+ */
+
+const RELAY_HTTP =
+  process.env.BUZZ_E2E_RELAY_URL ?? "https://sprout-oss.stage.blox.sqprod.co";
+const NSEC = process.env.BUZZ_PERF_NSEC ?? "";
+const COMMUNITY_HOST = process.env.BUZZ_COMMUNITY_HOST ?? "";
+const TARGET_CHANNEL = process.env.BUZZ_PERF_CHANNEL ?? "buzz-bugs";
+const SWIPES = Number(process.env.BUZZ_PERF_SWIPES ?? 30);
+const SAFE_MARGIN = 60;
+
+const IDENTITY_OVERRIDE_KEY = "buzz:e2e-identity-override.v1";
+const ONBOARDING_PREFIX = "buzz-onboarding-complete.v1:";
+const WELCOME_PREFIX = "buzz-welcome-channel-ensured.v2:";
+const REAL_CHROME_UA =
+  "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/148.0.0.0 Safari/537.36";
+
+test.use({ userAgent: REAL_CHROME_UA });
+
+function deriveIdentity(nsec: string) {
+  const decoded = decode(nsec.trim());
+  if (decoded.type !== "nsec") throw new Error("BUZZ_PERF_NSEC is not an nsec");
+  const skBytes = decoded.data as Uint8Array;
+  return {
+    privateKey: Buffer.from(skBytes).toString("hex"),
+    pubkey: getPublicKey(skBytes),
+    username: "perf-eva",
+  };
+}
+
+// One macOS-ish swipe: finger ramp (accelerating) + momentum tail (exp decay).
+function swipeDeltas(): number[] {
+  const deltas: number[] = [];
+  // finger: ~12 events ramping 4 -> 36 px
+  for (let i = 0; i < 12; i++) deltas.push(4 + Math.round((32 * i) / 11));
+  // momentum: decay from 36 by 0.94/event until < 1
+  let v = 36;
+  while (v >= 1) {
+    deltas.push(Math.round(v));
+    v *= 0.94;
+  }
+  return deltas; // ~68 events, ~1500px total
+}
+
+type Frame = {
+  t: number;
+  scrollTop: number;
+  rowId: string | null;
+  rowTop: number | null;
+  mounted: number;
+  fetch: number;
+};
+
+test("MEASURE: live trackpad-momentum upscroll profile", async ({ page }) => {
+  test.setTimeout(900_000);
+  if (!NSEC) throw new Error("Set BUZZ_PERF_NSEC to a real member nsec");
+  const identity = deriveIdentity(NSEC);
+
+  await installRelayBridge(page, "tyler");
+  const wsUrl = RELAY_HTTP.replace(/^http/, "ws");
+  await page.addInitScript(
+    ({ ident, onboardingPrefix, welcomePrefix, relayUrl, overrideKey }) => {
+      window.localStorage.setItem(overrideKey, JSON.stringify(ident));
+      window.localStorage.setItem(`${onboardingPrefix}${ident.pubkey}`, "true");
+      window.localStorage.setItem(
+        `${welcomePrefix}${encodeURIComponent(relayUrl)}:${ident.pubkey}`,
+        "true",
+      );
+      const w = window as unknown as { __BUZZ_E2E__?: Record<string, unknown> };
+      w.__BUZZ_E2E__ = { ...(w.__BUZZ_E2E__ ?? {}), identity: ident };
+    },
+    {
+      ident: identity,
+      onboardingPrefix: ONBOARDING_PREFIX,
+      welcomePrefix: WELCOME_PREFIX,
+      relayUrl: wsUrl,
+      overrideKey: IDENTITY_OVERRIDE_KEY,
+    },
+  );
+
+  const relayHost = new URL(RELAY_HTTP).host;
+  await page.route(
+    (url) => url.host === relayHost,
+    async (route) => {
+      const req = route.request();
+      const fwd = { ...req.headers(), "user-agent": REAL_CHROME_UA };
+      delete fwd["sec-ch-ua"];
+      delete fwd["sec-ch-ua-mobile"];
+      delete fwd["sec-ch-ua-platform"];
+      if (COMMUNITY_HOST) fwd.host = COMMUNITY_HOST;
+      let resp: Awaited<ReturnType<typeof route.fetch>> | null = null;
+      for (let attempt = 0; attempt < 5; attempt++) {
+        try {
+          resp = await route.fetch({ headers: fwd });
+          break;
+        } catch (err) {
+          if (attempt === 4) throw err;
+          await new Promise((r) => setTimeout(r, 1500));
+        }
+      }
+      if (!resp) throw new Error("unreachable");
+      const headers = { ...resp.headers() };
+      headers["access-control-allow-origin"] = "*";
+      headers["access-control-allow-headers"] = "*";
+      headers["access-control-allow-methods"] = "*";
+      await route.fulfill({ response: resp, headers, body: await resp.body() });
+    },
+  );
+
+  await page.goto("/");
+  await page.getByTestId("app-sidebar").waitFor({ state: "visible" });
+  const chan = page.getByTestId(`channel-${TARGET_CHANNEL}`).first();
+  await chan.waitFor({ state: "visible", timeout: 45_000 });
+  await chan.click();
+  await page
+    .locator('[data-testid="message-timeline"] [data-message-id]')
+    .first()
+    .waitFor({ state: "visible", timeout: 30_000 });
+  await page.waitForTimeout(3000);
+
+  const timeline = page.getByTestId("message-timeline");
+  // WKWebView mirror.
+  await timeline.evaluate((el) => {
+    (el as HTMLElement).style.overflowAnchor = "none";
+  });
+  await timeline.evaluate((element) => {
+    const el = element as HTMLDivElement;
+    el.scrollTop = el.scrollHeight;
+    el.dispatchEvent(new Event("scroll", { bubbles: true }));
+  });
+  await page.waitForTimeout(500);
+
+  // ---- Per-RAF sampler, in page, independent of input cadence ----
+  await timeline.evaluate((element, margin: number) => {
+    const el = element as HTMLDivElement;
+    const store = window as unknown as {
+      __FRAMES__: Frame[];
+      __SAMPLER_STOP__?: boolean;
+      __CHANNEL_WINDOW_FETCH_COUNT__?: number;
+    };
+    type Frame = {
+      t: number;
+      scrollTop: number;
+      rowId: string | null;
+      rowTop: number | null;
+      mounted: number;
+      fetch: number;
+    };
+    store.__FRAMES__ = [];
+    let trackedId: string | null = null;
+    const pick = (): string | null => {
+      const box = el.getBoundingClientRect();
+      const mid = box.top + box.height / 2;
+      let best: { id: string; d: number } | null = null;
+      for (const row of el.querySelectorAll<HTMLElement>("[data-message-id]")) {
+        const r = row.getBoundingClientRect();
+        if (r.top <= box.top + margin || r.bottom >= box.bottom - margin)
+          continue;
+        const d = Math.abs((r.top + r.bottom) / 2 - mid);
+        if (!best || d < best.d) best = { id: row.dataset.messageId ?? "", d };
+      }
+      return best?.id || null;
+    };
+    const loop = () => {
+      if (store.__SAMPLER_STOP__) return;
+      const box = el.getBoundingClientRect();
+      let rowTop: number | null = null;
+      if (trackedId) {
+        const row = el.querySelector<HTMLElement>(
+          `[data-message-id="${CSS.escape(trackedId)}"]`,
+        );
+        if (row) {
+          const r = row.getBoundingClientRect();
+          if (r.top > box.top + margin && r.bottom < box.bottom - margin)
+            rowTop = r.top;
+        }
+      }
+      if (rowTop === null) {
+        trackedId = pick();
+        if (trackedId) {
+          const r = el
+            .querySelector<HTMLElement>(
+              `[data-message-id="${CSS.escape(trackedId)}"]`,
+            )
+            ?.getBoundingClientRect();
+          rowTop = r ? r.top : null;
+        }
+      }
+      store.__FRAMES__.push({
+        t: performance.now(),
+        scrollTop: el.scrollTop,
+        rowId: trackedId,
+        rowTop,
+        mounted: el.querySelectorAll("[data-message-id]").length,
+        fetch: store.__CHANNEL_WINDOW_FETCH_COUNT__ ?? 0,
+      });
+      requestAnimationFrame(loop);
+    };
+    requestAnimationFrame(loop);
+  }, SAFE_MARGIN);
+
+  // ---- Trusted trackpad-like wheel input via CDP ----
+  const box = await timeline.boundingBox();
+  if (!box) throw new Error("no timeline box");
+  const cx = box.x + box.width / 2;
+  const cy = box.y + box.height / 2;
+  const isChromium =
+    page.context().browser()?.browserType().name() === "chromium";
+  const cdp = isChromium ? await page.context().newCDPSession(page) : null;
+  await page.mouse.move(cx, cy);
+
+  for (let s = 0; s < SWIPES; s++) {
+    const deltas = swipeDeltas();
+    for (const d of deltas) {
+      if (cdp) {
+        await cdp.send("Input.dispatchMouseEvent", {
+          type: "mouseWheel",
+          x: cx,
+          y: cy,
+          deltaX: 0,
+          deltaY: -d,
+          pointerType: "mouse",
+        });
+      } else {
+        await page.mouse.wheel(0, -d);
+      }
+      await new Promise((r) => setTimeout(r, 8));
+    }
+    // brief finger-off pause between swipes, like a real gesture chain
+    await page.waitForTimeout(120);
+    const at = await timeline.evaluate(
+      (el) => (el as HTMLDivElement).scrollTop,
+    );
+    if (at <= 0) {
+      // at the wall — give the prepend a chance to land, keep swiping after
+      await page.waitForTimeout(2500);
+    }
+    if (s % 5 === 4)
+      console.log(
+        `[swipe ${s + 1}/${SWIPES}] scrollTop=${at.toFixed(0)} frames=${await page.evaluate(() => (window as unknown as { __FRAMES__: unknown[] }).__FRAMES__.length)}`,
+      );
+  }
+
+  await page.evaluate(() => {
+    (window as unknown as { __SAMPLER_STOP__?: boolean }).__SAMPLER_STOP__ =
+      true;
+  });
+  const frames = (await page.evaluate(
+    () => (window as unknown as { __FRAMES__: Frame[] }).__FRAMES__,
+  )) as Frame[];
+
+  // ---- Analysis: per-frame deviation of row motion vs applied scroll ----
+  type Dev = {
+    i: number;
+    dev: number;
+    applied: number;
+    rowMove: number;
+    dt: number;
+    mountedGrew: boolean;
+    fetch: number;
+    scrollTop: number;
+  };
+  const devs: Dev[] = [];
+  for (let i = 1; i < frames.length; i++) {
+    const a = frames[i - 1];
+    const b = frames[i];
+    if (!a.rowId || a.rowId !== b.rowId) continue; // re-pick boundary
+    if (a.rowTop === null || b.rowTop === null) continue;
+    const applied = a.scrollTop - b.scrollTop;
+    const rowMove = b.rowTop - a.rowTop;
+    devs.push({
+      i,
+      dev: rowMove - applied,
+      applied,
+      rowMove,
+      dt: b.t - a.t,
+      mountedGrew: b.mounted > a.mounted,
+      fetch: b.fetch,
+      scrollTop: b.scrollTop,
+    });
+  }
+
+  const scored = devs.filter((d) => !d.mountedGrew); // prepend-commit frames reported separately
+  const commits = devs.filter((d) => d.mountedGrew);
+  const jumps = scored
+    .filter((d) => Math.abs(d.dev) > 2)
+    .sort((x, y) => Math.abs(y.dev) - Math.abs(x.dev));
+
+  const hist = new Map<number, number>();
+  for (const d of scored) {
+    const bucket =
+      Math.abs(d.dev) <= 1
+        ? 0
+        : Math.sign(d.dev) * 2 ** Math.ceil(Math.log2(Math.abs(d.dev)));
+    hist.set(bucket, (hist.get(bucket) ?? 0) + 1);
+  }
+
+  console.log(
+    `\n=== LIVE TRACKPAD PROFILE: #${TARGET_CHANNEL} engine=${page.context().browser()?.browserType().name()} ===`,
+  );
+  console.log(
+    `frames=${frames.length} scoredFramePairs=${scored.length} swipes=${SWIPES} fetchPages=${frames[frames.length - 1]?.fetch ?? 0} finalMounted=${frames[frames.length - 1]?.mounted ?? 0}`,
+  );
+  console.log(
+    "\n--- per-frame deviation histogram (0 = tracked row moved exactly what input asked; buckets are +/- powers of 2 px) ---",
+  );
+  for (const [k, v] of [...hist.entries()].sort((a, b) => a[0] - b[0]))
+    console.log(`  ${String(k).padStart(6)} : ${v}`);
+  console.log(
+    `\nsmooth frames (|dev|<=1px): ${scored.filter((d) => Math.abs(d.dev) <= 1).length}/${scored.length} = ${((100 * scored.filter((d) => Math.abs(d.dev) <= 1).length) / Math.max(1, scored.length)).toFixed(2)}%`,
+  );
+  console.log(`\n--- jump frames |dev|>2px: ${jumps.length} ---`);
+  for (const d of jumps.slice(0, 50))
+    console.log(
+      `  frame=${d.i} dev=${d.dev.toFixed(1)}px applied=${d.applied.toFixed(1)} rowMove=${d.rowMove.toFixed(1)} dt=${d.dt.toFixed(1)}ms fetch=${d.fetch} scrollTop=${d.scrollTop.toFixed(0)}`,
+    );
+  console.log(`\n--- prepend-commit frames (${commits.length}) ---`);
+  for (const d of commits)
+    console.log(
+      `  frame=${d.i} dev=${d.dev.toFixed(1)}px applied=${d.applied.toFixed(1)} rowMove=${d.rowMove.toFixed(1)} dt=${d.dt.toFixed(1)}ms fetch=${d.fetch} scrollTop=${d.scrollTop.toFixed(0)}`,
+    );
+
+  // long frames = jank of a different kind (main-thread stalls)
+  const longFrames = devs.filter((d) => d.dt > 34);
+  console.log(
+    `\n--- long frames (>34ms, i.e. dropped at least one 60Hz frame): ${longFrames.length} ---`,
+  );
+  for (const d of longFrames.slice(0, 25))
+    console.log(
+      `  frame=${d.i} dt=${d.dt.toFixed(0)}ms dev=${d.dev.toFixed(1)} applied=${d.applied.toFixed(1)} fetch=${d.fetch} scrollTop=${d.scrollTop.toFixed(0)}`,
+    );
+  console.log("==============================================\n");
+
+  expect(frames.length).toBeGreaterThan(500);
+});