diff --git a/apps/desktop/scripts/diag-drag-trace.mjs b/apps/desktop/scripts/diag-drag-trace.mjs
new file mode 100644
index 00000000000..01c4527fc06
--- /dev/null
+++ b/apps/desktop/scripts/diag-drag-trace.mjs
@@ -0,0 +1,217 @@
+// What is the drag actually spending time on?
+//
+// The render counters proved React is no longer the cost (commits 83 -> 12
+// after the $layoutTree fix) yet drag fps stayed ~3 while p95 halved. That
+// pattern says a fixed per-frame floor outside React. This takes a real CDP
+// trace of one sash drag and prints the category split — Recalculate Style,
+// Layout, Paint, Scripting — so the next fix targets the actual cost instead
+// of the next plausible-looking thing.
+//
+//   node scripts/diag-drag-trace.mjs [--port 9222] [--tiles 5]
+
+import { attach } from './perf/lib/launch.mjs'
+import { sleep } from './perf/lib/cdp.mjs'
+
+const arg = (name, fallback) => {
+  const i = process.argv.indexOf(`--${name}`)
+
+  return i === -1 ? fallback : process.argv[i + 1]
+}
+
+const port = Number(arg('port', 9222))
+const TILES = Number(arg('tiles', 5))
+const TURNS = Number(arg('turns', 20))
+
+const setup = `
+  (() => {
+    const hook = window.__HERMES_SESSION_TILES__
+    if (!hook) return 'no-hook'
+    const turn = (sid, i) => ([
+      { id: sid + '-u' + i, role: 'user', timestamp: Date.now(),
+        parts: [{ type: 'text', text: 'Question ' + i }] },
+      { id: sid + '-a' + i, role: 'assistant', timestamp: Date.now(), pending: false,
+        parts: [{ type: 'text', text: '## Finding ' + i + '\\n\\nProse with **bold** and \`code\`.\\n' }] }
+    ])
+    window.__T__ = { ids: [] }
+    for (let n = 1; n <= ${TILES}; n++) {
+      const sid = 'trace-tile-' + n
+      const rid = 'trace-rt-' + n
+      const messages = []
+      for (let i = 0; i < ${TURNS}; i++) messages.push(...turn(sid, i))
+      messages.push({ id: sid + '-stream', role: 'assistant', timestamp: Date.now(), pending: true,
+        parts: [{ type: 'text', text: 'Working.' }] })
+      window.__T__.ids.push({ sid, rid })
+      hook.open(sid, 'center')
+      hook.patch(sid, { runtimeId: rid })
+      hook.publish(rid, {
+        storedSessionId: sid, messages, branch: '', cwd: '', model: '', provider: '',
+        reasoningEffort: '', serviceTier: '', fast: false, yolo: false, personality: '',
+        busy: true, awaitingResponse: false, streamId: sid + '-stream', sawAssistantPayload: true,
+        pendingBranchGroup: null, interrupted: false, interimBoundaryPending: false,
+        needsInput: false, turnStartedAt: Date.now(), usage: null
+      })
+    }
+    return 'ok'
+  })()
+`
+
+const reveal = sid => `window.__HERMES_LAYOUT_TREE__.reveal(${JSON.stringify(`session-tile:${sid}`)})`
+
+// Drive the sash WITHOUT awaiting rAF: a slow app would stretch a rAF-paced
+// loop and make the window itself a function of the slowness. Fixed wall-clock
+// pacing keeps the trace window comparable run to run.
+const DRAG = `
+  (async () => {
+    const handle = document.querySelector('[role="separator"]')
+    if (!handle) return 'none'
+    const box = handle.getBoundingClientRect()
+    const y = box.top + box.height / 2
+    const x0 = box.left + box.width / 2
+    let x = x0
+    const opts = { bubbles: true, cancelable: true, pointerId: 1, pointerType: 'mouse', isPrimary: true, button: 0, buttons: 1 }
+    handle.dispatchEvent(new PointerEvent('pointerdown', { ...opts, clientX: x, clientY: y }))
+    for (let i = 0; i < 40; i++) {
+      x += (i < 20 ? 3 : -3)
+      window.dispatchEvent(new PointerEvent('pointermove', { ...opts, clientX: x, clientY: y }))
+      await new Promise(r => setTimeout(r, 16))
+    }
+    window.dispatchEvent(new PointerEvent('pointerup', { ...opts, buttons: 0, clientX: x, clientY: y }))
+    return 'dragged'
+  })()
+`
+
+const CLEANUP = `
+  (() => {
+    if (window.__T__) {
+      for (const { sid, rid } of window.__T__.ids) {
+        const s = window.__HERMES_SESSION_TILES__.states()
+        window.__HERMES_SESSION_TILES__.publish(rid, { ...s[rid], busy: false, streamId: null })
+        window.__HERMES_SESSION_TILES__.close(sid)
+      }
+      window.__T__ = null
+    }
+    return 'cleaned'
+  })()
+`
+
+const { cdp, teardown } = await attach({ port })
+
+try {
+  await cdp.send('Runtime.enable')
+
+  const ok = await cdp.eval(setup)
+
+  if (ok !== 'ok') {
+    throw new Error(`setup failed: ${ok}`)
+  }
+
+  for (let n = 1; n <= TILES; n++) {
+    await cdp.eval(reveal(`trace-tile-${n}`))
+    await sleep(300)
+  }
+
+  await sleep(1500)
+
+  // Collect trace events for the drag window only. `cdp.on` is the client's
+  // only event API (no `once`), so completion is signalled through a flag.
+  const events = []
+  let complete = false
+  cdp.on('Tracing.dataCollected', params => events.push(...(params.value ?? [])))
+  cdp.on('Tracing.tracingComplete', () => {
+    complete = true
+  })
+
+  await cdp.send('Tracing.start', {
+    transferMode: 'ReportEvents',
+    traceConfig: { includedCategories: ['devtools.timeline', 'blink.user_timing'] }
+  })
+
+  const dragged = await cdp.eval(DRAG)
+
+  await cdp.send('Tracing.end')
+
+  for (let waited = 0; !complete && waited < 10000; waited += 200) {
+    await sleep(200)
+  }
+
+  await cdp.eval(CLEANUP)
+
+  // Sum self-time per timeline category. Nested events would double-count, so
+  // attribute each event's duration minus the duration of its direct children.
+  const INTERESTING = new Set([
+    'UpdateLayoutTree', // Recalculate Style
+    'Layout',
+    'Paint',
+    'PaintImage',
+    'Layerize',
+    'UpdateLayer',
+    'CompositeLayers',
+    'FunctionCall',
+    'EvaluateScript',
+    'TimerFire',
+    'EventDispatch',
+    'HitTest',
+    'ParseHTML',
+    'CommitLoad'
+  ])
+
+  const totals = new Map()
+  let traced = 0
+
+  for (const e of events) {
+    if (e.ph !== 'X' || typeof e.dur !== 'number') {
+      continue
+    }
+
+    traced += 1
+    const name = e.name
+
+    if (!INTERESTING.has(name)) {
+      continue
+    }
+
+    totals.set(name, (totals.get(name) ?? 0) + e.dur / 1000)
+  }
+
+  console.log(`drag=${dragged}  trace events=${events.length} (complete=${traced})\n`)
+  console.log('TIMELINE COST (ms, total duration by event):')
+
+  const rows = [...totals.entries()].sort((a, b) => b[1] - a[1])
+
+  if (rows.length === 0) {
+    console.log('  (no timeline events — category filter or tracing domain unavailable)')
+  }
+
+  for (const [name, ms] of rows) {
+    console.log(`  ${name.padEnd(20)} ${ms.toFixed(1)}ms`)
+  }
+
+  const style = totals.get('UpdateLayoutTree') ?? 0
+  const layout = totals.get('Layout') ?? 0
+  const script = (totals.get('FunctionCall') ?? 0) + (totals.get('EvaluateScript') ?? 0) + (totals.get('TimerFire') ?? 0)
+
+  console.log(`\nVERDICT: style=${style.toFixed(0)}ms layout=${layout.toFixed(0)}ms script=${script.toFixed(0)}ms`)
+
+  // Script dominates -> name the functions. Timeline FunctionCall events carry
+  // the callsite in args.data, so the top offenders can be attributed without
+  // a separate CPU profile.
+  const byFn = new Map()
+
+  for (const e of events) {
+    if (e.ph !== 'X' || e.name !== 'FunctionCall' || typeof e.dur !== 'number') {
+      continue
+    }
+
+    const d = e.args?.data ?? {}
+    const key = `${d.functionName || '(anonymous)'} @ ${(d.url || '?').split('/').pop()}:${d.lineNumber ?? '?'}`
+    byFn.set(key, (byFn.get(key) ?? 0) + e.dur / 1000)
+  }
+
+  console.log('\nTOP SCRIPT CALLSITES (ms):')
+
+  for (const [name, ms] of [...byFn.entries()].sort((a, b) => b[1] - a[1]).slice(0, 15)) {
+    console.log(`  ${ms.toFixed(1).padStart(8)}  ${name}`)
+  }
+} finally {
+  teardown?.()
+}
diff --git a/apps/desktop/scripts/diag-ro-storm.mjs b/apps/desktop/scripts/diag-ro-storm.mjs
new file mode 100644
index 00000000000..f405aa65cd1
--- /dev/null
+++ b/apps/desktop/scripts/diag-ro-storm.mjs
@@ -0,0 +1,170 @@
+// How many ResizeObserver callbacks does one sash drag actually fire, and for
+// how many DISTINCT elements? The trace named use-resize-observer.ts at 977ms
+// but not whether that's a few expensive calls or a great many cheap ones —
+// and the fix differs completely between those.
+//
+//   node scripts/diag-ro-storm.mjs [--port 9222] [--tiles 5]
+
+import { attach } from './perf/lib/launch.mjs'
+import { sleep } from './perf/lib/cdp.mjs'
+
+const arg = (name, fallback) => {
+  const i = process.argv.indexOf(`--${name}`)
+
+  return i === -1 ? fallback : process.argv[i + 1]
+}
+
+const port = Number(arg('port', 9222))
+const TILES = Number(arg('tiles', 5))
+const TURNS = Number(arg('turns', 20))
+
+const setup = `
+  (() => {
+    const hook = window.__HERMES_SESSION_TILES__
+    if (!hook) return 'no-hook'
+    const turn = (sid, i) => ([
+      { id: sid + '-u' + i, role: 'user', timestamp: Date.now(),
+        parts: [{ type: 'text', text: 'Question ' + i + ' about the diff and its error path.' }] },
+      { id: sid + '-a' + i, role: 'assistant', timestamp: Date.now(), pending: false,
+        parts: [{ type: 'text', text: '## Finding ' + i + '\\n\\nProse with **bold** and \`code\`.\\n' }] }
+    ])
+    window.__R__ = { ids: [] }
+    for (let n = 1; n <= ${TILES}; n++) {
+      const sid = 'ro-tile-' + n
+      const rid = 'ro-rt-' + n
+      const messages = []
+      for (let i = 0; i < ${TURNS}; i++) messages.push(...turn(sid, i))
+      messages.push({ id: sid + '-stream', role: 'assistant', timestamp: Date.now(), pending: true,
+        parts: [{ type: 'text', text: 'Working.' }] })
+      window.__R__.ids.push({ sid, rid })
+      hook.open(sid, 'center')
+      hook.patch(sid, { runtimeId: rid })
+      hook.publish(rid, {
+        storedSessionId: sid, messages, branch: '', cwd: '', model: '', provider: '',
+        reasoningEffort: '', serviceTier: '', fast: false, yolo: false, personality: '',
+        busy: true, awaitingResponse: false, streamId: sid + '-stream', sawAssistantPayload: true,
+        pendingBranchGroup: null, interrupted: false, interimBoundaryPending: false,
+        needsInput: false, turnStartedAt: Date.now(), usage: null
+      })
+    }
+    return 'ok'
+  })()
+`
+
+const reveal = sid => `window.__HERMES_LAYOUT_TREE__.reveal(${JSON.stringify(`session-tile:${sid}`)})`
+
+// Patch ResizeObserver to count callbacks + distinct observed targets, then
+// drag and report. Counting happens in the page so nothing crosses CDP per call.
+//
+// NOTE: the app's shared observer (hooks/use-resize-observer.ts) is created
+// lazily on first use, so this patch must be installed BEFORE any surface
+// mounts — otherwise the shared instance is a native one this wrapper never
+// sees and every counter reads zero. `constructed` is the tell: a run showing
+// a handful of constructions and zero callbacks means the patch landed late,
+// not that the app stopped observing.
+const INSTRUMENT = `
+  (() => {
+    if (window.__ROSTATS__) return 'already'
+    const Native = window.ResizeObserver
+    const stats = { constructed: 0, observed: 0, callbacks: 0, entries: 0, targets: new Set(), on: false }
+    window.__ROSTATS__ = stats
+    window.ResizeObserver = class extends Native {
+      constructor(cb) {
+        super((entries, obs) => {
+          if (stats.on) {
+            stats.callbacks += 1
+            stats.entries += entries.length
+            for (const e of entries) stats.targets.add(e.target)
+          }
+          return cb(entries, obs)
+        })
+        stats.constructed += 1
+      }
+      observe(...args) {
+        stats.observed += 1
+        return super.observe(...args)
+      }
+    }
+    return 'patched'
+  })()
+`
+
+const DRAG = `
+  (async () => {
+    const s = window.__ROSTATS__
+    s.callbacks = 0; s.entries = 0; s.targets = new Set(); s.on = true
+    const handle = document.querySelector('[role="separator"]')
+    if (!handle) { s.on = false; return JSON.stringify({ error: 'no sash' }) }
+    const box = handle.getBoundingClientRect()
+    const y = box.top + box.height / 2
+    const x0 = box.left + box.width / 2
+    let x = x0
+    const opts = { bubbles: true, cancelable: true, pointerId: 1, pointerType: 'mouse', isPrimary: true, button: 0, buttons: 1 }
+    const t0 = performance.now()
+    handle.dispatchEvent(new PointerEvent('pointerdown', { ...opts, clientX: x, clientY: y }))
+    for (let i = 0; i < 40; i++) {
+      x += (i < 20 ? 3 : -3)
+      window.dispatchEvent(new PointerEvent('pointermove', { ...opts, clientX: x, clientY: y }))
+      await new Promise(r => setTimeout(r, 16))
+    }
+    window.dispatchEvent(new PointerEvent('pointerup', { ...opts, buttons: 0, clientX: x, clientY: y }))
+    await new Promise(r => setTimeout(r, 300))
+    s.on = false
+    return JSON.stringify({
+      ms: Math.round(performance.now() - t0),
+      moves: 40,
+      constructed: s.constructed,
+      observed: s.observed,
+      callbacks: s.callbacks,
+      entries: s.entries,
+      distinctTargets: s.targets.size,
+      userBubbles: document.querySelectorAll('[data-slot="aui_user-message-root"]').length
+    })
+  })()
+`
+
+const CLEANUP = `
+  (() => {
+    if (window.__R__) {
+      for (const { sid, rid } of window.__R__.ids) {
+        const s = window.__HERMES_SESSION_TILES__.states()
+        window.__HERMES_SESSION_TILES__.publish(rid, { ...s[rid], busy: false, streamId: null })
+        window.__HERMES_SESSION_TILES__.close(sid)
+      }
+      window.__R__ = null
+    }
+    return 'cleaned'
+  })()
+`
+
+const { cdp, teardown } = await attach({ port })
+
+try {
+  await cdp.send('Runtime.enable')
+  await cdp.eval(INSTRUMENT)
+
+  const ok = await cdp.eval(setup)
+
+  if (ok !== 'ok') {
+    throw new Error(`setup failed: ${ok}`)
+  }
+
+  for (let n = 1; n <= TILES; n++) {
+    await cdp.eval(reveal(`ro-tile-${n}`))
+    await sleep(300)
+  }
+
+  await sleep(1500)
+
+  const r = JSON.parse(await cdp.eval(DRAG))
+  await cdp.eval(CLEANUP)
+
+  console.log(JSON.stringify(r, null, 2))
+
+  if (r.moves) {
+    console.log(`\nper pointermove: ${(r.entries / r.moves).toFixed(1)} RO entries`)
+    console.log(`distinct elements resized: ${r.distinctTargets} (user bubbles in DOM: ${r.userBubbles})`)
+  }
+} finally {
+  teardown?.()
+}
diff --git a/apps/desktop/src/hooks/use-resize-observer.ts b/apps/desktop/src/hooks/use-resize-observer.ts
index 6ff08387ffc..61cd950a3d7 100644
--- a/apps/desktop/src/hooks/use-resize-observer.ts
+++ b/apps/desktop/src/hooks/use-resize-observer.ts
@@ -12,7 +12,65 @@ import { type RefObject, useLayoutEffect, useRef } from 'react'
  * instances mounting at once (every user bubble on a session switch), the
  * interleaved read→write→read pattern cascades into seconds of layout thrash.
  * Inside RO timing, layout is already clean and the same reads are ~free.
+ *
+ * ONE observer is shared by every caller. A private `new ResizeObserver` per
+ * hook instance means the browser delivers one callback PER CONSUMER when a
+ * common ancestor resizes, and each of those is a separate trip through the
+ * observer machinery. A single shared observer batches the same work into one
+ * delivery carrying many entries.
+ *
+ * Measured on a sash drag with five mounted session tiles (~100 user bubbles):
+ * 2,600 callbacks across 40 pointermoves — 65 separate callbacks per frame,
+ * each carrying exactly one entry — and 977ms of script time attributed to
+ * this file. Batching collapses that to one callback per frame.
  */
+
+type Handler = (entries: readonly ResizeObserverEntry[]) => void
+
+/** Live target → handler routing for the shared observer. */
+const handlers = new WeakMap<Element, Set<Handler>>()
+
+let shared: null | ResizeObserver = null
+
+function sharedObserver(): null | ResizeObserver {
+  if (typeof ResizeObserver === 'undefined') {
+    return null
+  }
+
+  if (!shared) {
+    shared = new ResizeObserver(entries => {
+      // Group this delivery's entries by handler so a caller observing several
+      // elements is still invoked once, with all of its entries — the same
+      // contract a private observer gave it.
+      const byHandler = new Map<Handler, ResizeObserverEntry[]>()
+
+      for (const entry of entries) {
+        const targets = handlers.get(entry.target)
+
+        if (!targets) {
+          continue
+        }
+
+        for (const handler of targets) {
+          const list = byHandler.get(handler)
+
+          if (list) {
+            list.push(entry)
+          } else {
+            byHandler.set(handler, [entry])
+          }
+        }
+      }
+
+      for (const [handler, group] of byHandler) {
+        handler(group)
+      }
+    })
+  }
+
+  return shared
+}
+
 export function useResizeObserver(
   onResize: (entries: readonly ResizeObserverEntry[]) => void,
   ...refs: readonly RefObject<Element | null>[]
@@ -21,14 +79,15 @@ export function useResizeObserver(
   refsRef.current = refs
 
   useLayoutEffect(() => {
-    if (typeof ResizeObserver === 'undefined') {
+    const observer = sharedObserver()
+
+    if (!observer) {
       onResize([])
 
       return
     }
 
-    const observer = new ResizeObserver(entries => onResize(entries))
-    let observed = false
+    const observed: Element[] = []
 
     for (const ref of refsRef.current) {
       const element = ref.current
@@ -37,16 +96,39 @@ export function useResizeObserver(
         continue
       }
 
-      observer.observe(element)
-      observed = true
+      const existing = handlers.get(element)
+
+      if (existing) {
+        existing.add(onResize)
+      } else {
+        handlers.set(element, new Set([onResize]))
+        // Only the first handler for an element needs to register it; the
+        // observer fires once per element regardless of how many care.
+        observer.observe(element)
+      }
+
+      observed.push(element)
     }
 
-    if (!observed) {
-      observer.disconnect()
-
+    if (observed.length === 0) {
       return
     }
 
-    return () => observer.disconnect()
+    return () => {
+      for (const element of observed) {
+        const set = handlers.get(element)
+
+        if (!set) {
+          continue
+        }
+
+        set.delete(onResize)
+
+        if (set.size === 0) {
+          handlers.delete(element)
+          observer.unobserve(element)
+        }
+      }
+    }
   }, [onResize])
 }