diff --git a/apps/web/tests/notice-wake-tail.e2e.ts b/apps/web/tests/notice-wake-tail.e2e.ts
index 085e6e8a4b..884adcb9d3 100644
--- a/apps/web/tests/notice-wake-tail.e2e.ts
+++ b/apps/web/tests/notice-wake-tail.e2e.ts
@@ -21,15 +21,11 @@ const NOTICE_TURN = HISTORY_TURNS + 2
 /** Tool Steps in the finished Turn; their folded height must exceed the notice Turn's height. */
 const TOOL_STEPS = 12
 const OUTPUT_LINES = 40
-/** A settled tail leaves only composer padding uncovered (about 55-105 px at this viewport). */
-const BLANK_LIMIT = 150
 /** The recorded host gap between the waking claim and Step 1, spent assembling the system prompt. */
 const PRE_STEP_GAP_MS = 187
 const REPLY = 'NOTICE_WAKE_REPLY'
 
 interface Tail {
-  /** Viewport pixels above the composer that no painted row covers. */
-  readonly blank: number
   /** Height of the room reserved below the flow. */
   readonly spacer: number
   readonly noticeVisible: boolean
@@ -49,22 +45,8 @@ function readTail(page: Page): Promise<Tail> {
       const rect = element.getBoundingClientRect()
       return rect.height > 0.5 && rect.bottom > viewTop && rect.top < viewBottom
     }
-    const spans = [...host.querySelectorAll<HTMLElement>('[data-chat-anchor-key]')]
-      .map(row => row.getBoundingClientRect())
-      .filter(rect => rect.height > 0.5)
-      .map(rect => [Math.max(rect.top, viewTop), Math.min(rect.bottom, viewBottom)] as const)
-      .filter(([top, bottom]) => bottom > top)
-      .sort((left, right) => left[0] - right[0])
-    let covered = 0
-    let reach = viewTop
-    for (const [top, bottom] of spans) {
-      if (bottom <= reach) continue
-      covered += bottom - Math.max(top, reach)
-      reach = bottom
-    }
     const spacer = host.querySelector<HTMLElement>('[data-chat-turn-spacer]')
     return {
-      blank: Math.round(viewBottom - viewTop - covered),
       spacer: spacer === null ? 0 : Number.parseFloat(spacer.style.height) || 0,
       noticeVisible: [...host.querySelectorAll('[data-turn-trigger]')].some(inView),
       replyVisible: [...host.querySelectorAll('[data-chat-flow-kind="assistant-step"]')]
@@ -159,7 +141,7 @@ describe('web e2e: notice Turn after a long Detailed Turn', () => {
         session.append('step/end', { turn: LONG_TURN, step })
         session.append('step/start', { turn: LONG_TURN, step: step + 1 })
         await scaffold.ctx.sessions.flush(session)
-        await page.waitForTimeout(30)
+        await page.locator(`[data-chat-node-key$="${callId}"]`).waitFor({ state: 'attached', timeout: 15_000 })
       }
       session.append('assistant/message', {
         stream: [],
@@ -177,7 +159,7 @@ describe('web e2e: notice Turn after a long Detailed Turn', () => {
         const host = document.querySelector<HTMLElement>('[data-conversation-scroll]')
         if (host !== null) host.scrollTop = host.scrollHeight
       })
-      await page.waitForTimeout(600)
+      await page.locator('[data-chat-following-tail]').waitFor({ timeout: 10_000 })
 
       // The Turn ends; 15 ms later (the recorded production gap) a settled subagent wakes the next Turn.
       session.append('turn/end', { turn: LONG_TURN, reason: { kind: 'completed' } })
@@ -199,12 +181,14 @@ describe('web e2e: notice Turn after a long Detailed Turn', () => {
       session.append('agent/inbox/spliced', { target: 'next-turn', start: 0, removedCount: 1, inserted: [] })
       setRunning(true)
       await scaffold.ctx.sessions.flush(session)
+      // The browser has applied the notice Turn's start before the gap is held and sampled.
+      await page.locator(`[data-chat-turn="${String(NOTICE_TURN)}"]`).first().waitFor({ state: 'attached', timeout: 15_000 })
       await page.waitForTimeout(PRE_STEP_GAP_MS)
       const gap = await readTail(page)
       session.append('step/start', { turn: NOTICE_TURN, step: 1 })
       session.append('user/message', notice, { surfaceOp: 'append' })
       await scaffold.ctx.sessions.flush(session)
-      await page.waitForTimeout(300)
+      await page.locator('[data-turn-trigger]').first().waitFor({ state: 'attached', timeout: 15_000 })
       session.append('assistant/message', {
         stream: [],
         turn: NOTICE_TURN,
@@ -216,11 +200,15 @@ describe('web e2e: notice Turn after a long Detailed Turn', () => {
       }, { surfaceOp: 'append' })
       await scaffold.ctx.sessions.flush(session)
 
-      await page.waitForTimeout(3000)
-      const tail = await readTail(page)
       expect(gap).toMatchObject({ longTurnOpen: true, spacer: 0 })
-      expect(tail).toMatchObject({ noticeVisible: true, replyVisible: true, longTurnOpen: true })
-      expect(tail.blank).toBeLessThan(BLANK_LIMIT)
+      const settled = { noticeVisible: true, replyVisible: true, longTurnOpen: true, spacer: 0 }
+      await expect.poll(() => readTail(page), { timeout: 15_000 }).toMatchObject(settled)
+      // The settled tail survives later frames instead of passing through on the way to a fold.
+      await page.evaluate(() => new Promise<void>((resolve) => {
+        requestAnimationFrame(() => { requestAnimationFrame(() => { resolve() }) })
+      }))
+      expect(await readTail(page)).toMatchObject(settled)
+      expect(tripwire.warnings).toEqual([])
       expect(tripwire.pageErrors).toEqual([])
     } finally {
       await page.context().close()
diff --git a/docs/i18n/terminology.md b/docs/i18n/terminology.md
index cf7009ff7f..5971f88600 100644
--- a/docs/i18n/terminology.md
+++ b/docs/i18n/terminology.md
@@ -127,6 +127,7 @@
 | event log | 事件日志 | | | |
 | event stream | 事件流 | | | |
 | event-sourced | 事件溯源 | | | 沿用 DDD 社区通行译法 |
+| exchange | 交互段 | | 回合、交换 | Chat 中从真人输入开启的轮次起、到下一个这样的轮次为止的连续轮次；见术语表 |
 | Executive summary | 摘要 | | | 事故复盘标题用语 |
 | executor | 执行器 | | | |
 | expected output | 预期输出 | | 金标 | 指 snapshot 比较产物；翻译语料的人工校准样例不在此列 |
diff --git a/packages/client/ui-chat/src/client/conversation-nodes/README.i18n.yaml b/packages/client/ui-chat/src/client/conversation-nodes/README.i18n.yaml
index 922219671d..64ed556c17 100644
--- a/packages/client/ui-chat/src/client/conversation-nodes/README.i18n.yaml
+++ b/packages/client/ui-chat/src/client/conversation-nodes/README.i18n.yaml
@@ -34,7 +34,7 @@
   zh: 0ab47a4014ea5e1e
 /chat-conversation-node-rules/display-modes/collapse-timing:
   en: 03d744dd1c92bdd3
-  zh: b26e0fca325b47c5
+  zh: 19804eaf3234632a
 /chat-conversation-node-rules/display-modes/group-title-rules:
   en: 2392290662bc69e8
   zh: 8872d651c07a8f4c
diff --git a/packages/client/ui-chat/src/client/conversation-nodes/README.zh.md b/packages/client/ui-chat/src/client/conversation-nodes/README.zh.md
index 72ba992f79..089a0b886a 100644
--- a/packages/client/ui-chat/src/client/conversation-nodes/README.zh.md
+++ b/packages/client/ui-chat/src/client/conversation-nodes/README.zh.md
@@ -138,7 +138,7 @@ description: "Chat 过程分组与活动摘要的明细业务规则。"
 
 设置 → 通用设置 → 过程收起时机位于工作步骤展示正下方，与展示模式独立。「完成时收起」是默认值，保留立即折叠和即时跟尾定位；「下次发送时收起」启用延后折叠及其协调过渡。选择以 `ui-chat.collapseTiming` 保存到与工作步骤展示相同的 Host profile 设置，刷新或重新打开应用后会恢复。未保存此值的 profile 使用「完成时收起」，不会自动写入默认值。此偏好不写入浏览器存储或 Session 数据。
 
-选择「下次发送时收起」后，最新交互段完成时保留运行中的呈现：简洁和标准保留过程组头，详细保留平铺过程行。交互段从首个 Step 接收真人输入的轮次开始，包含之后的所有轮次，直到下一个这样的轮次，因此由通知（子 agent 完成、goal 轮次、定时任务或后台作业）唤醒的轮次不会收起同一交互段里更早的轮次。普通 transcript 输入进入待发送状态，或出现开启更新交互段的轮次时，取消这项例外，但保留用户对整轮的手动开合选择；仅出现新轮次的开始不会取消，因为 Host 先认领唤醒输入，再记录 Step 1。只有外层过程实际隐藏时，才重置内部开合。排队输入和单独的 steering 不会取消例外。重新打开 Session 时，其最新交互段也享有例外，已加载窗口中没有开启轮次时从第一个已加载轮次算起；整轮的手动开合选择仍优先。切回「完成时收起」会立即取消例外。
+选择「下次发送时收起」后，最新交互段完成时保留运行中的呈现：简洁和标准保留过程组头，详细保留平铺过程行。交互段从首个 Step 接收真人输入的轮次开始，包含之后的所有轮次，直到下一个这样的轮次，因此由通知（子 agent 完成、Goal Round、定时任务或后台作业）唤醒的轮次不会收起同一交互段里更早的轮次。普通 transcript 输入进入待发送状态，或出现开启更新交互段的轮次时，取消这项例外，但保留用户对整轮的手动开合选择；仅出现新轮次的开始不会取消，因为 Host 先认领唤醒输入，再记录 Step 1。只有外层过程实际隐藏时，才重置内部开合。排队输入和单独的 steering 不会取消例外。重新打开 Session 时，其最新交互段也享有例外，已加载窗口中没有开启轮次时从第一个已加载轮次算起；整轮的手动开合选择仍优先。切回「完成时收起」会立即取消例外。
 
 新消息触发折叠时，过程收起与跟随新消息合为一次向上运动，折叠结束后不再追加滚动。屏幕外的步骤不增加底部补白；只有保留的正文和新消息无法填满可见内容移动所需空间时，才保留必要差额。折叠期间新增的内容会更新剩余运动；重新展开全部正在收起的行时，取消该运动并释放失效的底部补白。阅读手势会中断自动跟随，折叠结束不会重新接管。
 
diff --git a/packages/client/ui-chat/src/client/conversation-nodes/chat-snapshot-builder.ts b/packages/client/ui-chat/src/client/conversation-nodes/chat-snapshot-builder.ts
index 4bd8ce2704..dad0b6e51e 100644
--- a/packages/client/ui-chat/src/client/conversation-nodes/chat-snapshot-builder.ts
+++ b/packages/client/ui-chat/src/client/conversation-nodes/chat-snapshot-builder.ts
@@ -12,6 +12,7 @@ import type {
   ChatTurnNavigationIndex, ChatTurnProcessPresentation, LegacyConversationSlice, TurnNavigationItem,
 } from '../contract/snapshot.ts'
 import { TURN_PROCESS_INDEPENDENT_KINDS } from '../contract/turn-process.ts'
+import { locationCoordinates } from './common.ts'
 import { sessionRecallLabels, skillInvocationName } from './event-projection.ts'
 import { GroupTreeSegments } from './group-tree-segments.ts'
 import { sameTurnNavigationItem, turnNavigationItem } from './turn-navigation.ts'
@@ -415,12 +416,6 @@ function stepKey(turn: number, step: number): string {
   return `${turn}:${step}`
 }
 
-function locationCoordinates(location: ConversationLocation): { turn?: number; step?: number } {
-  if (location.kind === 'step') return { turn: location.turn.turn, step: location.step.step }
-  if (location.kind === 'turn') return { turn: location.turn.turn }
-  return {}
-}
-
 function locationTurnStatus(location: ConversationLocation): string | undefined {
   return location.kind === 'turn' || location.kind === 'step' ? location.turn.status : undefined
 }
diff --git a/packages/client/ui-chat/src/client/conversation-nodes/common.ts b/packages/client/ui-chat/src/client/conversation-nodes/common.ts
index 55e2bb6a8b..ac7859271b 100644
--- a/packages/client/ui-chat/src/client/conversation-nodes/common.ts
+++ b/packages/client/ui-chat/src/client/conversation-nodes/common.ts
@@ -29,6 +29,17 @@ export function contextLocation(context: ConversationNodeContext): ConversationL
   return context.start?.location ?? context.matches[0]?.location ?? { kind: 'unresolved' }
 }
 
+/**
+ * Read the Turn and Step a Location resolves to.
+ * @param location - engine-owned Location.
+ * @returns resolved coordinates; Session-level and unresolved Locations have none.
+ */
+export function locationCoordinates(location: ConversationLocation): { turn?: number; step?: number } {
+  if (location.kind === 'step') return { turn: location.turn.turn, step: location.step.step }
+  if (location.kind === 'turn') return { turn: location.turn.turn }
+  return {}
+}
+
 /**
  * Build one final Chat target Node with the engine-owned stable key.
  * @param context - assembled business Context.
diff --git a/packages/client/ui-chat/src/client/conversation-nodes/group-tree-segments.ts b/packages/client/ui-chat/src/client/conversation-nodes/group-tree-segments.ts
index 2628bb3f6c..738b79ef29 100644
--- a/packages/client/ui-chat/src/client/conversation-nodes/group-tree-segments.ts
+++ b/packages/client/ui-chat/src/client/conversation-nodes/group-tree-segments.ts
@@ -1,5 +1,6 @@
 import type { ChatConversationViewNode } from '../contract/chat-nodes.ts'
 import { isAtOrOutside, type ChatGroupTree } from '../contract/group-tree.ts'
+import { locationCoordinates } from './common.ts'
 
 const NO_TURNS: readonly number[] = []
 
@@ -7,7 +8,9 @@ const NO_TURNS: readonly number[] = []
  * Loaded Turns split into consecutive segments at the Turns holding a Node at
  * one group-tree position, visible or hidden. The latest segment starts at the
  * last such Turn, or at the first loaded Turn when none is loaded. Node changes
- * update per-Turn counts, so no operation scans the loaded Nodes.
+ * update per-Turn counts without scanning the loaded Nodes; `settle` returns at
+ * once while the start holds and otherwise reads loaded Turns only from the
+ * earlier of the previous and new starts.
  */
 export class GroupTreeSegments {
   /** Opening Node count by owning Turn. */
@@ -28,7 +31,7 @@ export class GroupTreeSegments {
     const after = this.openingTurn(next)
     if (before === after) return
     if (before !== undefined && this.adjust(before, -1) === 0 && before === this.latestOpening) {
-      this.latestOpening = this.openings.size === 0 ? undefined : Math.max(...this.openings.keys())
+      this.latestOpening = this.lastOpeningTurn()
     }
     if (after !== undefined) {
       this.adjust(after, 1)
@@ -67,9 +70,15 @@ export class GroupTreeSegments {
   }
 
   private openingTurn(node: ChatConversationViewNode | undefined): number | undefined {
-    if (node === undefined || !isAtOrOutside(node, this.position)) return undefined
-    const location = node.location
-    return location.kind === 'turn' || location.kind === 'step' ? location.turn.turn : undefined
+    return node !== undefined && isAtOrOutside(node, this.position) ? locationCoordinates(node.location).turn : undefined
+  }
+
+  private lastOpeningTurn(): number | undefined {
+    let last: number | undefined
+    for (const turn of this.openings.keys()) {
+      if (last === undefined || turn > last) last = turn
+    }
+    return last
   }
 
   /** @returns the Turn's opening count after the change; zero removes the Turn. */
diff --git a/packages/client/ui-chat/src/client/conversation-nodes/message.ts b/packages/client/ui-chat/src/client/conversation-nodes/message.ts
index 27ad1c07e2..d5ef45f93c 100644
--- a/packages/client/ui-chat/src/client/conversation-nodes/message.ts
+++ b/packages/client/ui-chat/src/client/conversation-nodes/message.ts
@@ -125,7 +125,7 @@ export const messageDefinition: ConversationNodeDefinition<MessageNode> = {
       waking ? 'turn-trigger' : context.state.kind,
       context.state.seq,
       context.state,
-      { groupTree: opensExchange ? 'exchange' : undefined },
+      { location, groupTree: opensExchange ? 'exchange' : undefined },
     )
   },
 }
diff --git a/packages/client/ui-chat/tests/group-tree-segments.client.spec.ts b/packages/client/ui-chat/tests/group-tree-segments.client.spec.ts
index 8e092e7633..377ca9c7d2 100644
--- a/packages/client/ui-chat/tests/group-tree-segments.client.spec.ts
+++ b/packages/client/ui-chat/tests/group-tree-segments.client.spec.ts
@@ -78,6 +78,21 @@ describe('Chat group tree', () => {
     expect(members(segments, order)).toEqual(order)
   })
 
+  it('falls back to the latest remaining opening Turn whatever order the openings arrived in', () => {
+    const segments = new GroupTreeSegments('exchange')
+    const order = [1, 2, 3, 4, 5]
+    const latest = node('latest', 5, 'exchange')
+    for (const value of [node('middle', 3, 'exchange'), node('earliest', 1, 'exchange'), latest]) {
+      segments.track(undefined, value)
+    }
+    segments.settle(order)
+    expect(members(segments, order)).toEqual([5])
+
+    segments.track(latest, undefined)
+    expect(segments.settle(order)).toEqual([3, 4])
+    expect(members(segments, order)).toEqual([3, 4, 5])
+  })
+
   it('opens segments at Nodes of the configured position or an outer one', () => {
     const segments = new GroupTreeSegments('turn')
     const order = [1, 2, 3]