Make chat spacer follow the latest visible user message

ARCHITECTURE.md

@@ -503,7 +503,7 @@ automatically armed by installing Möbius.
 
 ## Chat scroll + steer contract
 
-**Owner-authoritative contract — v1.6 (2026-07-22).** This section is the
+**Owner-authoritative contract — v1.7 (2026-07-24).** This section is the
 canonical source of truth for how a chat scrolls and steers. When implementation,
 comments, and this contract disagree, the implementation/comments are the bug:
 fix behavior to match this contract. If a real case is unspecified or the desired
@@ -527,12 +527,19 @@ and attaches their rule ids to new diagnostic chats. The Playwright lock-in spec
   waits for the spacer-exhaustion handoff. A viewport /
   keyboard change, foreground return, mount, or chat restoration must never create
   auto-scroll.
-- **R1 — Permanent exact reservation.** Every non-empty chat keeps enough dynamic
-  bottom spacer for its latest visible user message to reach the viewport top,
-  including after leaving and reopening the chat. The reservation is exact — no
-  extra scrollable blank beyond that target — and shrinks as the reply fills it.
-  `FOLLOW_BOTTOM` follows real conversation content, excluding the reservation, so
-  a short restored chat cannot open on an empty viewport.
+- **R1 — Visible latest-user reservation.** Dynamic bottom spacer belongs only to
+  the latest visible user row. It reserves exactly enough room for that row to reach
+  the viewport top and shrinks as reply, tool, image, or other content fills the
+  deficit. The remaining room survives turn completion when the reply is short.
+  Expanding content consumes it; collapsing the same content restores the exact
+  deficit. Mode does not own its lifetime: `PIN_USER_MSG` may reserve before a fresh
+  row is placed, while `ANCHOR_AT`, `FOLLOW_BOTTOM`, mount/return, question
+  submission, and disclosure settlement reserve only when their real viewport
+  contains the latest user row. If that row leaves the viewport, spacer is zero;
+  seeing an older user row never qualifies. An otherwise unreachable anchor clamps
+  to real conversation content before visibility is decided. Positive dynamic
+  spacer therefore always implies that the latest user row is visible at the
+  current/applied target.
 - **R2 — One send rule everywhere.** The first visible user message always pins to
   the viewport top. Every subsequent direct, queued, promoted, or steered message
   pins when its submit-time DOM snapshot is at the real-content tail. Geometry is
@@ -569,6 +576,9 @@ and attaches their rule ids to new diagnostic chats. The Playwright lock-in spec
   manufacture a top-of-chat location or engage live following. That automatic tail
   fallback is not a reader-chosen location and must not be persisted on pagehide or
   shell reload; only a deliberate scroll/send/pagination position earns restoration.
+  Exactness is bounded by real content: if a viewport growth or content collapse
+  makes the saved target unreachable, clamp it to the nearest real conversation
+  position, then apply R1 only if that viewport shows the latest user row.
 - **R5 — Reader owns gestures and layout-only sends.** From the first wheel/touch/key
   input until its scroll event lands, no layout path may write `scrollTop`: stream
   resize, spacer handoff, terminal promotion, catch-up, and viewport/keyboard resize
@@ -608,11 +618,11 @@ and attaches their rule ids to new diagnostic chats. The Playwright lock-in spec
   card enters its pending state or output resumes, the controller snapshots the
   currently visible message and its exact viewport offset as `ANCHOR_AT`. Resumed
   output grows without dragging the reader, even when the chat had been following
-  the tail before Submit. If a mobile viewport grows before that output arrives,
-  the dynamic spacer temporarily reserves enough room to keep the anchor target
-  reachable; the reservation disappears as real content replaces it. A failed
-  answer keeps that settled reading anchor for the retryable card rather than
-  manufacturing follow intent again.
+  the tail before Submit. The answer anchor does not independently own spacer; it
+  receives R1's exact deficit only while its viewport still shows the latest user
+  row. If a viewport growth makes its exact offset unreachable, it clamps to real
+  conversation content. A failed answer keeps that settled reading anchor for the
+  retryable card rather than manufacturing follow intent again.
   The source handoff
   preserves the question, its answer, and every pre/post-answer thinking, tool, and
   text block in event order, without hiding, duplicating, or reordering them. Only a
@@ -633,14 +643,15 @@ path means routing it through the same entries rather than inventing another rul
 | First direct/queued/steered user row becomes visible | any | `PIN_USER_MSG` | New row to top |
 | Later send submitted at real-content tail (mode may be one frame stale) | any | `PIN_USER_MSG` | New row to top |
 | Later send submitted anywhere else | hold or stale follow | `ANCHOR_AT`/existing hold | None |
-| Reader reaches physical bottom while live reservation remains | any | armed `PIN_USER_MSG` | User-owned; then keep prompt fixed |
-| Reader reaches physical bottom while idle reservation remains | any | settled `PIN_USER_MSG` | User-owned; keep prompt fixed |
+| Reader reaches physical bottom while latest-user reservation remains and turn is live | any | armed `PIN_USER_MSG` | User-owned; then keep prompt fixed |
+| Reader reaches physical bottom while latest-user reservation remains and turn is idle | any | settled `PIN_USER_MSG` | User-owned; keep prompt fixed |
 | Reader reaches bottom with no reservation remaining | any | `FOLLOW_BOTTOM` | User-owned |
 | Reader scrolls manually away from bottom | any | `ANCHOR_AT` | User-owned |
 | Reply grows while an armed live pin still has reserved room | pin hold | same pin hold | Keep prompt fixed |
 | Streaming reply consumes the armed pin reservation | pin hold | `FOLLOW_BOTTOM` | Follow real-content tail |
 | Short reply settles before consuming the reservation | armed pin hold | settled pin hold | Keep prompt fixed; retire automatic handoff |
-| Other layout grows while pinned or anchored | hold | same hold | Reapply only the held target |
+| Other layout grows/collapses while latest user is visible | any hold | same hold | Consume/restore exact R1 deficit |
+| Latest user leaves the viewport | any | same reader mode | Collapse spacer to zero |
 | Viewport/keyboard changes | `PIN_USER_MSG` | same `PIN_USER_MSG` | Reapply pin after resize; never infer intent from keyboard-open geometry |
 | Viewport/keyboard changes | follow or anchor hold | same follow if still at tail, otherwise hold anchor | Never creates follow |
 | Chat exits/backgrounds/returns | any | `ANCHOR_AT` | Restore exact saved anchor |
@@ -656,6 +667,10 @@ Controller structure is part of the contract, not an implementation detail:
 - Every live mode mutation goes through `transitionMode`; every mode-owned
   `scrollTop` write goes through `writeMode`. The exported `applyMode` executor
   is for the controller and pure unit tests, not a second live writer.
+- `useScrollMode` is the sole writer of `.spacer-dynamic` height. The write is
+  derived from the latest user row's visibility and exact content deficit;
+  disclosure helpers and renderers may preserve an on-screen anchor but may never
+  prime, enlarge, or unwind spacer themselves.
 - The gesture-gated `scroll` event reads physical-bottom geometry directly.
   Do not reintroduce a sentinel or asynchronous observer as a second bottom
   authority: its delayed state can contradict the viewport that caused the
@@ -679,8 +694,9 @@ but never backward. Do not derive a remounted timer solely from `Date.now()` or
 client arrival time of replayed deltas: catch-up arrives as a burst and that makes a
 minutes-old turn visibly restart at one second.
 
-Every visible user row also makes R1's reservation current, whether or not that row
-pins. Reservation lifetime and pin decisions are independent.
+Only the latest visible user row makes R1's reservation current. Reservation
+lifetime follows that row's visibility and exact remaining deficit, not turn
+completion or a particular scroll mode.
 - **A restored send is one logical message.** The frontend scopes the draft
   identity to the chat and reuses its client-minted `cid` when an ambiguous
   failed POST restores an unchanged composer. The route checks that durable

frontend/src/components/ChatView/__tests__/anchorHeldThroughToggle.test.js

@@ -4,41 +4,37 @@ import assert from 'node:assert/strict'
 import { anchorHeldThroughToggle } from '../chatContract.js'
 
 // The disclosure seam (thinking/tool/activity collapse) is where the chat UI
-// regressed repeatedly: a bounce (anchor top moved across the toggle) or spacer
-// over-reservation (rapid toggles stacked body heights). This predicate is the
-// machine-checkable law for that interaction; these tests pin its behavior so a
-// browser monitor or Playwright spec can trust it.
+// regressed repeatedly: a bounce (anchor top moved across the toggle) or blank
+// room without the latest user row. This predicate is the machine-checkable
+// law for an ANCHOR_AT toggle.
 
 test('a held anchor with no spacer growth passes', () => {
   const r = anchorHeldThroughToggle(
-    { anchorTop: 120, spacerH: 200 },
-    { anchorTop: 121, spacerH: 200 },
-    { bodyH: 0 })
+    { anchorTop: 120, spacerH: 0 },
+    { anchorTop: 121, spacerH: 0 })
   assert.equal(r.ok, true)
 })
 
 test('an anchor that bounced past tolerance fails', () => {
   const r = anchorHeldThroughToggle(
-    { anchorTop: 120, spacerH: 200 },
-    { anchorTop: 140, spacerH: 200 })
+    { anchorTop: 120, spacerH: 0 },
+    { anchorTop: 140, spacerH: 0 })
   assert.equal(r.ok, false)
   assert.equal(r.measured.drift, 20)
 })
 
-test('a collapse may grow the spacer by exactly one body height', () => {
-  const held = anchorHeldThroughToggle(
-    { anchorTop: 500, spacerH: 100 },
-    { anchorTop: 500, spacerH: 148 },
-    { bodyH: 48 })
-  assert.equal(held.ok, true)
+test('an anchored disclosure cannot leave room without the latest user visible', () => {
+  const reserved = anchorHeldThroughToggle(
+    { anchorTop: 500, spacerH: 0 },
+    { anchorTop: 500, spacerH: 48 })
+  assert.equal(reserved.ok, false)
 })
 
-test('a spacer that stacked two body heights (rapid-toggle regression) fails', () => {
-  const stacked = anchorHeldThroughToggle(
-    { anchorTop: 500, spacerH: 100 },
-    { anchorTop: 500, spacerH: 196 }, // baseline 100 + 48 + 48
-    { bodyH: 48 })
-  assert.equal(stacked.ok, false)
+test('collapse may restore reservation when the latest user remains visible', () => {
+  const reserved = anchorHeldThroughToggle(
+    { anchorTop: 500, spacerH: 0 },
+    { anchorTop: 500, spacerH: 48, latestUserVisible: true })
+  assert.equal(reserved.ok, true)
 })
 
 test('missing anchor data is indeterminate, never a false pass', () => {
@@ -47,12 +43,9 @@ test('missing anchor data is indeterminate, never a false pass', () => {
   assert.match(r.reason, /anchorTop/)
 })
 
-test('an explicit spacerBaseline overrides before.spacerH', () => {
-  // A caller that primed the spacer before measuring `before` passes the true
-  // pre-toggle baseline so the accumulation check is against the right value.
+test('a stale pre-toggle spacer passes only when the toggle leaves none behind', () => {
   const r = anchorHeldThroughToggle(
-    { anchorTop: 300, spacerH: 250 }, // already primed
     { anchorTop: 300, spacerH: 250 },
-    { bodyH: 48, spacerBaseline: 202 })
+    { anchorTop: 300, spacerH: 0 })
   assert.equal(r.ok, true)
 })

frontend/src/components/ChatView/__tests__/chatContract.test.js

@@ -67,17 +67,22 @@ test('ChatView only consumes methods returned by the scroll controller', () => {
     `ChatView consumes missing useScrollMode members: ${missing.join(', ')}`)
 })
 
-test('owner contract freezes question answers without changing active-row ownership', () => {
+test('owner contract freezes question answers without changing row or spacer ownership', () => {
   const architecture = readFileSync(
     new URL('../../../../../ARCHITECTURE.md', import.meta.url),
     'utf8',
   )
-  assert.match(architecture, /Owner-authoritative contract — v1\.6 \(2026-07-22\)/)
+  assert.match(architecture, /Owner-authoritative contract — v1\.7 \(2026-07-24\)/)
   assert.match(
     architecture,
     /In-process question is answered \| any \| `ANCHOR_AT` on current visible row; same active assistant row/,
     'question submission must freeze the reader while preserving the R6 row',
   )
+  assert.match(
+    architecture,
+    /The answer anchor does not independently own spacer/,
+    'question submission must use the latest-user visibility rule',
+  )
 })
 
 test('a retained chat crosses the old unmount lifecycle while hidden', () => {
@@ -389,8 +394,8 @@ test('every predicate id is registered in CHAT_CONTRACT (registry is the map)',
     cushionPresent(snapshotChatUX(pinnedEnv)),
     singleAssistantSurface(1),
     anchorHeldThroughToggle(
-      { anchorTop: 100, spacerH: 40 },
-      { anchorTop: 100, spacerH: 40 },
+      { anchorTop: 100, spacerH: 0 },
+      { anchorTop: 100, spacerH: 0 },
     ),
   ]
   for (const r of emitted) {

frontend/src/components/ChatView/__tests__/preserveTogglePosition.test.js

@@ -124,19 +124,20 @@ test('FOLLOW_BOTTOM leaves toggle movement entirely to the scroll controller', (
   }
 })
 
-test('closing a disclosure primes the bottom spacer before React removes its body', () => {
+test('disclosure preservation never reads or writes the dynamic spacer', () => {
   const originalMutationObserver = globalThis.MutationObserver
   const originalRaf = globalThis.requestAnimationFrame
-  const originalGetComputedStyle = globalThis.getComputedStyle
   globalThis.MutationObserver = undefined
   globalThis.requestAnimationFrame = () => 1
-  globalThis.getComputedStyle = () => ({ marginTop: '4px', marginBottom: '2px' })
 
   try {
-    const spacer = { offsetHeight: 100, style: {} }
+    let queried = false
     const scroller = {
       scrollTop: 50,
-      querySelector: selector => selector === '.spacer-dynamic' ? spacer : null,
+      querySelector: () => {
+        queried = true
+        throw new Error('disclosures do not own reservation geometry')
+      },
     }
     const body = { getBoundingClientRect: () => ({ height: 60 }) }
     const anchor = {
@@ -148,127 +149,10 @@ test('closing a disclosure primes the bottom spacer before React removes its bod
     }
 
     preserveTogglePosition(anchor)
-    assert.equal(spacer.style.height, '166px')
+    assert.equal(queried, false)
   }
   finally {
     globalThis.MutationObserver = originalMutationObserver
     globalThis.requestAnimationFrame = originalRaf
-    globalThis.getComputedStyle = originalGetComputedStyle
-  }
-})
-
-test('opening a disclosure leaves normal spacer sizing alone', () => {
-  const originalMutationObserver = globalThis.MutationObserver
-  const originalRaf = globalThis.requestAnimationFrame
-  globalThis.MutationObserver = undefined
-  globalThis.requestAnimationFrame = () => 1
-
-  try {
-    const spacer = { offsetHeight: 100, style: {} }
-    const scroller = {
-      scrollTop: 50,
-      querySelector: () => spacer,
-    }
-    const anchor = {
-      parentElement: {},
-      nextElementSibling: null,
-      closest: () => scroller,
-      getAttribute: () => 'false',
-      getBoundingClientRect: () => ({ top: 120 }),
-    }
-
-    preserveTogglePosition(anchor)
-    assert.equal(spacer.style.height, undefined)
-  }
-  finally {
-    globalThis.MutationObserver = originalMutationObserver
-    globalThis.requestAnimationFrame = originalRaf
-  }
-})
-
-test('a fast close-open-close cycle unwinds provisional space instead of accumulating it', () => {
-  const originalMutationObserver = globalThis.MutationObserver
-  const originalRaf = globalThis.requestAnimationFrame
-  const originalGetComputedStyle = globalThis.getComputedStyle
-  globalThis.MutationObserver = undefined
-  globalThis.requestAnimationFrame = () => 1
-  globalThis.getComputedStyle = () => ({ marginTop: '4px', marginBottom: '2px' })
-
-  try {
-    const spacer = {
-      style: { height: '100px' },
-      get offsetHeight() { return Number.parseFloat(this.style.height) },
-    }
-    const scroller = {
-      scrollTop: 50,
-      querySelector: () => spacer,
-    }
-    const body = { getBoundingClientRect: () => ({ height: 60 }) }
-    let expanded = 'true'
-    const anchor = {
-      parentElement: {},
-      nextElementSibling: body,
-      closest: () => scroller,
-      getAttribute: () => expanded,
-      getBoundingClientRect: () => ({ top: 120 }),
-    }
-
-    preserveTogglePosition(anchor)
-    assert.equal(spacer.style.height, '166px')
-
-    // Re-open before ResizeObserver has replaced the provisional value.
-    expanded = 'false'
-    preserveTogglePosition(anchor)
-    assert.equal(spacer.style.height, '100px')
-
-    // A second fast close reserves one body height, not two.
-    expanded = 'true'
-    preserveTogglePosition(anchor)
-    assert.equal(spacer.style.height, '166px')
-  }
-  finally {
-    globalThis.MutationObserver = originalMutationObserver
-    globalThis.requestAnimationFrame = originalRaf
-    globalThis.getComputedStyle = originalGetComputedStyle
-  }
-})
-
-test('provisional cleanup never overwrites a newer authoritative spacer value', () => {
-  const originalMutationObserver = globalThis.MutationObserver
-  const originalRaf = globalThis.requestAnimationFrame
-  const originalGetComputedStyle = globalThis.getComputedStyle
-  globalThis.MutationObserver = undefined
-  globalThis.requestAnimationFrame = () => 1
-  globalThis.getComputedStyle = () => ({ marginTop: '0px', marginBottom: '0px' })
-
-  try {
-    const spacer = {
-      style: { height: '100px' },
-      get offsetHeight() { return Number.parseFloat(this.style.height) },
-    }
-    const scroller = { scrollTop: 0, querySelector: () => spacer }
-    const body = { getBoundingClientRect: () => ({ height: 60 }) }
-    let expanded = 'true'
-    const anchor = {
-      parentElement: {},
-      nextElementSibling: body,
-      closest: () => scroller,
-      getAttribute: () => expanded,
-      getBoundingClientRect: () => ({ top: 80 }),
-    }
-
-    preserveTogglePosition(anchor)
-    assert.equal(spacer.style.height, '160px')
-
-    // Simulate ResizeObserver publishing new settled geometry.
-    spacer.style.height = '112px'
-    expanded = 'false'
-    preserveTogglePosition(anchor)
-    assert.equal(spacer.style.height, '112px')
-  }
-  finally {
-    globalThis.MutationObserver = originalMutationObserver
-    globalThis.requestAnimationFrame = originalRaf
-    globalThis.getComputedStyle = originalGetComputedStyle
   }
 })