perf(desktop): add a session-load scenario to the harness

`submit` measures the scroll jump when a turn is appended; nothing measured
the jump when a session is opened, which is the prepend/settle path. Clicks
sidebar rows and tracks how far the bottom turn moves after first paint.

apps/desktop/scripts/perf/README.md

@@ -57,6 +57,7 @@ directly via `window.__PERF_DRIVE__`, so no LLM credits are spent.
 | `first-token` | backend | Enter → first assistant token painted (TTFT) | (new) |
 | `submit` | backend | Enter → cleared → user msg painted, scroll jump | measure-submit, measure-jump |
 | `session-switch` | backend | route → first-paint → settle | profile-session-switch |
+| `session-load` | backend | how far a session's transcript moves after first paint | (new) |
 | `profile-switch` | backend | rail click → sidebar settled | measure-profile-switch |
 
 `ci` + `cold` scenarios need no backend/credits and are gated against

apps/desktop/scripts/perf/scenarios/index.mjs

@@ -8,6 +8,7 @@ import keystroke from './keystroke.mjs'
 import multitab from './multitab.mjs'
 import profileSwitch from './profile-switch.mjs'
 import renderChurn from './render-churn.mjs'
+import sessionLoad from './session-load.mjs'
 import sessionSwitch from './session-switch.mjs'
 import stream from './stream.mjs'
 import streamHistory from './stream-history.mjs'
@@ -25,6 +26,7 @@ export const SCENARIOS = {
   [coldStart.name]: coldStart,
   [firstToken.name]: firstToken,
   [submit.name]: submit,
+  [sessionLoad.name]: sessionLoad,
   [sessionSwitch.name]: sessionSwitch,
   [profileSwitch.name]: profileSwitch
 }

apps/desktop/scripts/perf/scenarios/session-load.mjs

@@ -0,0 +1,136 @@
+// Visual stability of a session LOAD. Sibling to `submit` (which measures the
+// jump on Enter); this one measures the jump on opening a session — the
+// prepend/settle path, not the append path.
+//
+// Clicks sidebar rows and tracks the bottom-most turn's on-screen top every
+// frame. A clean load never moves it after first paint; a janky one strands it
+// thousands of px away while the render-budget backfill and stick-to-bottom
+// argue. Backend tier: needs real stored sessions in the sidebar.
+//
+//   node scripts/perf/run.mjs session-load --rows 2,5,8 --rounds 2
+
+import { SELECTORS, sleep } from '../lib/cdp.mjs'
+import { summarize } from '../lib/stats.mjs'
+
+// Below this a shift is sub-perceptual (sub-pixel rounding, a settling caret).
+const SHIFT_PX = 4
+
+const ARM = `
+  (() => {
+    const samples = []
+    const t0 = performance.now()
+    let running = true
+
+    const tick = () => {
+      if (!running) return
+      const v = document.querySelector(${JSON.stringify(SELECTORS.threadViewport)})
+
+      if (v) {
+        const turns = v.querySelectorAll(
+          ${JSON.stringify(SELECTORS.turnPair)} + ',' + ${JSON.stringify(SELECTORS.assistantMessage)}
+        )
+        const last = turns[turns.length - 1]
+        const rect = last && last.getBoundingClientRect()
+        samples.push({
+          st: Math.round(v.scrollTop),
+          sh: v.scrollHeight,
+          ch: v.clientHeight,
+          bottomTop: rect ? Math.round(rect.top - v.getBoundingClientRect().top) : null,
+          turns: turns.length,
+          t: Math.round(performance.now() - t0)
+        })
+      }
+
+      requestAnimationFrame(tick)
+    }
+
+    requestAnimationFrame(tick)
+    window.__SL = { samples, stop() { running = false } }
+  })()
+`
+
+const CLICK = index => `
+  (() => {
+    const rows = [...document.querySelectorAll(${JSON.stringify(SELECTORS.rowButton)})].filter(el => el.offsetParent)
+    const row = rows[${index}]
+    if (!row) return null
+    row.click()
+    return (row.textContent ?? '').slice(0, 34)
+  })()
+`
+
+/** Total/max on-screen movement of the bottom turn after it first paints. */
+function measureLoad(samples) {
+  const painted = samples.findIndex(s => s.turns > 0)
+  const after = painted === -1 ? [] : samples.slice(painted)
+  const load = { maxShiftPx: 0, offBottomFrames: 0, settledMs: 0, shiftedPx: 0, shifts: 0 }
+  let previous = null
+
+  for (const sample of after) {
+    if (sample.sh - (sample.st + sample.ch) > 2) {
+      load.offBottomFrames += 1
+    }
+
+    if (previous?.bottomTop != null && sample.bottomTop != null) {
+      const delta = Math.abs(sample.bottomTop - previous.bottomTop)
+
+      if (delta > SHIFT_PX) {
+        load.maxShiftPx = Math.max(load.maxShiftPx, delta)
+        load.settledMs = sample.t
+        load.shiftedPx += delta
+        load.shifts += 1
+      }
+    }
+
+    previous = sample
+  }
+
+  return load
+}
+
+export default {
+  name: 'session-load',
+  tier: 'backend',
+  description: 'Visual stability of opening a session: how far the transcript moves after first paint.',
+  async run(cdp, opts = {}) {
+    const rows = String(opts.rows ?? '2,5,8').split(',').map(Number)
+    const rounds = Number(opts.rounds ?? 2)
+    const watchMs = Number(opts.watchMs ?? 4500)
+
+    await cdp.send('Runtime.enable')
+
+    const loads = []
+
+    for (let round = 0; round < rounds; round++) {
+      for (const index of rows) {
+        await cdp.eval(ARM)
+
+        if (!(await cdp.eval(CLICK(index)))) {
+          continue
+        }
+
+        await sleep(watchMs)
+        const { samples } = await cdp.eval('(() => { window.__SL.stop(); return window.__SL })()')
+        loads.push(measureLoad(samples))
+      }
+
+      await sleep(500)
+    }
+
+    if (!loads.length) {
+      throw new Error('session-load found no sidebar rows to click')
+    }
+
+    const total = key => loads.reduce((sum, load) => sum + load[key], 0)
+
+    return {
+      metrics: {
+        load_max_shift_px: Math.max(...loads.map(load => load.maxShiftPx)),
+        load_off_bottom_frames: Math.round(total('offBottomFrames') / loads.length),
+        load_settled_p95_ms: summarize(loads.map(load => load.settledMs)).p95,
+        load_shifted_px: Math.round(total('shiftedPx') / loads.length)
+      },
+      detail: { loads: loads.length, shiftsPerLoad: total('shifts') / loads.length }
+    }
+  }
+}