fix(app): keep live session updates incremental

Preserve unchanged message identities and isolate Vue row rendering during live refreshes.

Use a tail-only scroll path so updates do not interrupt active readers or expanded tools. Add regression coverage for 908-message appends, changed snapshots, and scroll policy.

app/src/renderer/src/data.js

@@ -242,7 +242,7 @@ export async function loadSessionDetail(sessionId) {
   const assembled = {
     ...(session || {}),
     id: sessionId,
-    messages: assembledMessages
+    messages: markRaw(assembledMessages)
   };
 
   if (workflow) {

app/src/renderer/src/session-timeline.mjs

@@ -0,0 +1,66 @@
+function sameSnapshotValue(current, incoming) {
+  if (Object.is(current, incoming)) return true;
+  if (current === null || incoming === null) return false;
+  if (typeof current !== 'object' || typeof incoming !== 'object') return false;
+
+  const currentIsArray = Array.isArray(current);
+  if (currentIsArray !== Array.isArray(incoming)) return false;
+
+  const currentKeys = Object.keys(current);
+  const incomingKeys = Object.keys(incoming);
+  if (currentKeys.length !== incomingKeys.length) return false;
+
+  for (const key of currentKeys) {
+    if (!Object.hasOwn(incoming, key)) return false;
+    if (!sameSnapshotValue(current[key], incoming[key])) return false;
+  }
+  return true;
+}
+
+/**
+ * Reconcile a complete timeline snapshot while preserving the identity of
+ * messages whose rendered content did not change.
+ */
+export function applySnapshot(current = [], incoming = []) {
+  const currentByUuid = new Map(
+    current
+      .filter(message => message?.uuid)
+      .map(message => [message.uuid, message]),
+  );
+  const incomingUuids = new Set(
+    incoming.filter(message => message?.uuid).map(message => message.uuid),
+  );
+  const addedIds = [];
+  const updatedIds = [];
+  const messages = incoming.map((message, index) => {
+    const uuid = message?.uuid;
+    const existing = uuid ? currentByUuid.get(uuid) : current[index];
+    if (!existing || (uuid && existing.uuid !== uuid)) {
+      if (uuid) addedIds.push(uuid);
+      return message;
+    }
+    if (sameSnapshotValue(existing, message)) return existing;
+    if (uuid) updatedIds.push(uuid);
+    return message;
+  });
+  const removedIds = current
+    .filter(message => message?.uuid && !incomingUuids.has(message.uuid))
+    .map(message => message.uuid);
+  const changed = messages.length !== current.length
+    || messages.some((message, index) => message !== current[index]);
+  const unchangedPrefix = incoming.length > current.length
+    && current.every((message, index) => messages[index] === message);
+  const tailOnly = changed
+    && unchangedPrefix
+    && updatedIds.length === 0
+    && removedIds.length === 0;
+
+  return {
+    messages: changed ? messages : current,
+    addedIds,
+    updatedIds,
+    removedIds,
+    changed,
+    tailOnly,
+  };
+}

app/src/renderer/src/session-view-state.mjs

@@ -9,10 +9,19 @@ function scrollItems(detail) {
   return arrayFrom(detail?.querySelectorAll?.(SCROLL_ITEM_SELECTOR));
 }
 
+export function isFollowingSessionTail(wrap, bottomThreshold = 50) {
+  if (!wrap) return false;
+  return wrap.scrollHeight - wrap.scrollTop - wrap.clientHeight < bottomThreshold;
+}
+
+export function restoreSessionTail({ wrap, followTail, restoreScroll = true } = {}) {
+  if (!wrap || !followTail || !restoreScroll) return;
+  wrap.scrollTop = wrap.scrollHeight;
+}
+
 export function captureSessionViewState({ wrap, detail, bottomThreshold = 50 } = {}) {
   if (!wrap) return null;
-  const distanceFromBottom = wrap.scrollHeight - wrap.scrollTop - wrap.clientHeight;
-  const followTail = distanceFromBottom < bottomThreshold;
+  const followTail = isFollowingSessionTail(wrap, bottomThreshold);
   const wrapTop = wrap.getBoundingClientRect?.().top || 0;
   const anchorElement = followTail
     ? null
@@ -62,7 +71,7 @@ export function restoreSessionViewState(snapshot, { wrap, detail, restoreScroll
   if (!restoreScroll) return;
 
   if (snapshot.followTail) {
-    wrap.scrollTop = wrap.scrollHeight;
+    restoreSessionTail({ wrap, followTail: true });
     return;
   }
 
@@ -77,19 +86,6 @@ export function restoreSessionViewState(snapshot, { wrap, detail, restoreScroll
   wrap.scrollTop += currentOffset - snapshot.anchor.offset;
 }
 
-export function reconcileSessionMessages(current = [], incoming = []) {
-  const currentByUuid = new Map(
-    current.filter(message => message?.uuid).map(message => [message.uuid, message]),
-  );
-  return incoming.map(message => {
-    if (!message?.uuid) return message;
-    const existing = currentByUuid.get(message.uuid);
-    if (!existing) return message;
-    Object.assign(existing, message);
-    return existing;
-  });
-}
-
 export function findLastMessageAtOrAbove(messages, bottomLine) {
   if (!messages?.length) return -1;
   let low = 0;

app/src/renderer/src/views/SessionDetail.vue

@@ -1,15 +1,17 @@
 <script setup>
-import { ref, computed, onMounted, onUnmounted, nextTick, onActivated, onDeactivated, watch } from 'vue';
+import { ref, shallowRef, computed, onMounted, onUnmounted, nextTick, onActivated, onDeactivated, watch } from 'vue';
 import { useRouter, useRoute } from 'vue-router';
 import { state, FOLDER_SVG } from '../store.js';
 import { loadSessionDetail, isTextTruncated, loadFullText } from '../data.js';
 import { clearSessionDirty, consumeGlobalSessionDirty } from '../session-live.mjs';
+import { applySnapshot } from '../session-timeline.mjs';
 import { getArgPreview, getToolIcon, renderTerminalTool } from '../tool-renderer.js';
 import FlapNumber from '../components/FlapNumber.vue';
 import {
   captureSessionViewState,
   findLastMessageAtOrAbove,
-  reconcileSessionMessages,
+  isFollowingSessionTail,
+  restoreSessionTail,
   restoreSessionViewState,
 } from '../session-view-state.mjs';
 import {
@@ -28,7 +30,7 @@ const route = useRoute();
 
 // --- Reactive state ---
 const session = computed(() => state.sessions.find(s => s.id === props.id));
-const messages = ref([]);
+const messages = shallowRef([]);
 const loading = ref(false);
 const progressPct = ref(0);
 const active = ref(false);
@@ -145,34 +147,56 @@ watch(() => props.id, async (newId, oldId) => {
 async function loadMessages({ force = false } = {}) {
   if (!props.id) return;
   const hadContent = messages.value.length > 0;
-  const viewState = hadContent
-    ? captureSessionViewState({ wrap: wrapRef.value, detail: detailRef.value })
-    : null;
-  const scrollRevisionAtLoad = scrollRevision;
+  let reconciliation;
+  let viewState = null;
+  let followTailBeforePatch = false;
+  let scrollRevisionBeforePatch = scrollRevision;
 
   loading.value = !hadContent;
   try {
     const s = state.sessions.find(x => x.id === props.id);
+    let latest = s;
     if (s && (force || !s.messages || s.messages.length === 0)) {
-      await loadSessionDetail(props.id);
+      latest = await loadSessionDetail(props.id);
     }
-    const latest = state.sessions.find(x => x.id === props.id);
     const incoming = latest?.messages || [];
-    messages.value = hadContent
-      ? reconcileSessionMessages(messages.value, incoming)
-      : incoming;
+    reconciliation = applySnapshot(messages.value, incoming);
+    if (reconciliation.changed) {
+      followTailBeforePatch = hadContent && isFollowingSessionTail(wrapRef.value);
+      scrollRevisionBeforePatch = scrollRevision;
+      if (hadContent && !reconciliation.tailOnly) {
+        viewState = captureSessionViewState({
+          wrap: wrapRef.value,
+          detail: detailRef.value,
+        });
+      }
+      messages.value = reconciliation.messages;
+    }
   } finally {
     loading.value = false;
   }
 
+  if (!reconciliation.changed) {
+    if (state.pendingFocusUuid) await focusPendingMessage();
+    return;
+  }
+
   await nextTick();
   syncTotalMessages();
   if (!state.pendingFocusUuid) {
-    restoreSessionViewState(viewState, {
-      wrap: wrapRef.value,
-      detail: detailRef.value,
-      restoreScroll: scrollRevision === scrollRevisionAtLoad,
-    });
+    if (reconciliation.tailOnly) {
+      restoreSessionTail({
+        wrap: wrapRef.value,
+        followTail: followTailBeforePatch,
+        restoreScroll: scrollRevision === scrollRevisionBeforePatch,
+      });
+    } else {
+      restoreSessionViewState(viewState, {
+        wrap: wrapRef.value,
+        detail: detailRef.value,
+        restoreScroll: scrollRevision === scrollRevisionBeforePatch,
+      });
+    }
     onScroll();
   }
 
@@ -554,17 +578,8 @@ function toggleRaw(event) {
   btn?.classList.toggle('active', showing);
 }
 
-function getSkillMd(skillMsgIdx) {
-  const msg = messages.value[skillMsgIdx];
-  if (msg?._skillMd) return msg._skillMd;
-  // Fallback: search next few messages
-  for (let i = skillMsgIdx + 1; i < Math.min(skillMsgIdx + 3, messages.value.length); i++) {
-    const m = messages.value[i];
-    if (m.is_meta === 1 && m.text && m.text.includes('Base directory for this skill')) {
-      return m.text;
-    }
-  }
-  return null;
+function getSkillMd(msg) {
+  return msg?._skillMd || null;
 }
 
 function toggleSkillMd(event) {
@@ -623,7 +638,7 @@ function getToolCallParsedInput(tc) {
 
         <!-- Message timeline -->
         <div class="timeline">
-          <template v-for="(msg, idx) in messages" :key="msg.uuid || idx">
+          <template v-for="msg in messages" :key="msg.uuid" v-memo="[msg, state.query]">
 
             <!-- Meta messages: collapsed system indicator -->
             <template v-if="msg.is_meta === 1">
@@ -738,12 +753,12 @@ function getToolCallParsedInput(tc) {
                     <span class="skill-card-name">{{ getToolCallParsedInput(msg.tool_calls[0]).skill || '?' }}</span>
                   </div>
                   <div class="skill-card-args">{{ getToolCallParsedInput(msg.tool_calls[0]).args || '' }}</div>
-                  <div v-if="getSkillMd(idx)" class="skill-card-md">
+                  <div v-if="getSkillMd(msg)" class="skill-card-md">
                     <button class="skill-md-toggle" @click="toggleSkillMd">
                       <svg class="chevron" viewBox="0 0 8 8" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round"><path d="M2.5 1.5l3 2.5-3 2.5"/></svg>
                       <span>SKILL.md</span>
                     </button>
-                    <div class="skill-md-body" v-html="renderMarkdown(getSkillMd(idx), { variant: 'compact' })"></div>
+                    <div class="skill-md-body" v-html="renderMarkdown(getSkillMd(msg), { variant: 'compact' })"></div>
                   </div>
                 </div>
               </div>

tests/session-timeline.test.mjs

@@ -0,0 +1,102 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+import { applySnapshot } from '../app/src/renderer/src/session-timeline.mjs';
+
+test('tail append reuses 908 existing messages without writing to them', () => {
+  let oldMessageWrites = 0;
+  const current = Array.from({ length: 908 }, (_, index) => new Proxy({
+    uuid: `message-${index}`,
+    type: index % 2 === 0 ? 'user' : 'assistant',
+    text: `message ${index}`,
+  }, {
+    set(target, property, value) {
+      oldMessageWrites++;
+      return Reflect.set(target, property, value);
+    },
+  }));
+  const incoming = [
+    ...current.map(message => ({ ...message })),
+    { uuid: 'message-908', type: 'assistant', text: 'new tail' },
+  ];
+
+  const result = applySnapshot(current, incoming);
+
+  assert.equal(result.messages.length, 909);
+  for (let index = 0; index < current.length; index++) {
+    assert.equal(result.messages[index], current[index]);
+  }
+  assert.equal(oldMessageWrites, 0);
+  assert.deepEqual(result.addedIds, ['message-908']);
+  assert.deepEqual(result.updatedIds, []);
+  assert.deepEqual(result.removedIds, []);
+  assert.equal(result.changed, true);
+  assert.equal(result.tailOnly, true);
+});
+
+test('a completed tool result replaces only its owning message', () => {
+  const first = { uuid: 'message-1', type: 'user', text: 'run it' };
+  const second = {
+    uuid: 'message-2',
+    type: 'assistant',
+    tool_calls: [{ id: 'call-1', name: 'Bash', result: null }],
+  };
+  const third = { uuid: 'message-3', type: 'assistant', text: 'waiting' };
+  const incomingSecond = {
+    uuid: 'message-2',
+    type: 'assistant',
+    tool_calls: [{ id: 'call-1', name: 'Bash', result: { content: 'done' } }],
+  };
+
+  const result = applySnapshot(
+    [first, second, third],
+    [{ ...first }, incomingSecond, { ...third }],
+  );
+
+  assert.equal(result.messages[0], first);
+  assert.equal(result.messages[1], incomingSecond);
+  assert.equal(result.messages[2], third);
+  assert.deepEqual(result.updatedIds, ['message-2']);
+  assert.deepEqual(result.addedIds, []);
+  assert.deepEqual(result.removedIds, []);
+  assert.equal(result.changed, true);
+  assert.equal(result.tailOnly, false);
+});
+
+test('an identical snapshot returns the original array', () => {
+  const current = [
+    { uuid: 'message-1', text: 'same', tool_calls: [{ id: 'call-1' }] },
+    { uuid: 'message-2', text: 'same too' },
+  ];
+
+  const result = applySnapshot(current, structuredClone(current));
+
+  assert.equal(result.messages, current);
+  assert.deepEqual(result.addedIds, []);
+  assert.deepEqual(result.updatedIds, []);
+  assert.deepEqual(result.removedIds, []);
+  assert.equal(result.changed, false);
+  assert.equal(result.tailOnly, false);
+});
+
+test('removals and reorders preserve matching identities without claiming a tail append', () => {
+  const first = { uuid: 'message-1', text: 'first' };
+  const second = { uuid: 'message-2', text: 'second' };
+  const removed = applySnapshot([first, second], [{ ...second }]);
+
+  assert.deepEqual(removed.removedIds, ['message-1']);
+  assert.equal(removed.messages[0], second);
+  assert.equal(removed.tailOnly, false);
+
+  const reordered = applySnapshot(
+    [first, second],
+    [{ ...second }, { ...first }],
+  );
+  assert.equal(reordered.messages[0], second);
+  assert.equal(reordered.messages[1], first);
+  assert.deepEqual(reordered.addedIds, []);
+  assert.deepEqual(reordered.updatedIds, []);
+  assert.deepEqual(reordered.removedIds, []);
+  assert.equal(reordered.changed, true);
+  assert.equal(reordered.tailOnly, false);
+});

tests/session-view-state.test.mjs

@@ -5,7 +5,8 @@ import { readFileSync } from 'node:fs';
 import {
   captureSessionViewState,
   findLastMessageAtOrAbove,
-  reconcileSessionMessages,
+  isFollowingSessionTail,
+  restoreSessionTail,
   restoreSessionViewState,
 } from '../app/src/renderer/src/session-view-state.mjs';
 
@@ -113,6 +114,33 @@ test('session refresh follows appended content only when already at the tail', (
   assert.equal(newWrap.scrollTop, 2400);
 });
 
+test('tail-follow detection uses only the scroll container metrics', () => {
+  assert.equal(isFollowingSessionTail(wrap({
+    scrollTop: 1360,
+    scrollHeight: 2000,
+    clientHeight: 600,
+  })), true);
+  assert.equal(isFollowingSessionTail(wrap({
+    scrollTop: 1200,
+    scrollHeight: 2000,
+    clientHeight: 600,
+  })), false);
+});
+
+test('tail append preserves an active reader and follows only without newer scroll input', () => {
+  const reader = wrap({ scrollTop: 600, scrollHeight: 2400, clientHeight: 600 });
+  restoreSessionTail({ wrap: reader, followTail: false });
+  assert.equal(reader.scrollTop, 600);
+
+  const userMoved = wrap({ scrollTop: 800, scrollHeight: 2400, clientHeight: 600 });
+  restoreSessionTail({ wrap: userMoved, followTail: true, restoreScroll: false });
+  assert.equal(userMoved.scrollTop, 800);
+
+  const follower = wrap({ scrollTop: 1400, scrollHeight: 2400, clientHeight: 600 });
+  restoreSessionTail({ wrap: follower, followTail: true });
+  assert.equal(follower.scrollTop, 2400);
+});
+
 test('session refresh never restores an old anchor over newer user scrolling', () => {
   const oldTool = disclosure('tool:call-1', ['open']);
   const snapshot = captureSessionViewState({
@@ -132,22 +160,6 @@ test('session refresh never restores an old anchor over newer user scrolling', (
   assert.equal(userScrolledWrap.scrollTop, 800, 'newer user scroll wins over stale refresh state');
 });
 
-test('message reconciliation preserves existing identities and appends the tail', () => {
-  const first = { uuid: 'm1', text: 'old', tool_calls: [{ id: 't1' }] };
-  const current = [first];
-  const incoming = [
-    { uuid: 'm1', text: 'updated', tool_calls: [{ id: 't1', result: { content: 'progress' } }] },
-    { uuid: 'm2', text: 'new tail' },
-  ];
-
-  const reconciled = reconcileSessionMessages(current, incoming);
-
-  assert.equal(reconciled[0], first);
-  assert.equal(reconciled[0].text, 'updated');
-  assert.equal(reconciled[0].tool_calls[0].result.content, 'progress');
-  assert.equal(reconciled[1], incoming[1]);
-});
-
 test('scroll progress locates the visible message without scanning the full session', () => {
   let layoutReads = 0;
   const messages = Array.from({ length: 2048 }, (_, index) => ({
@@ -161,17 +173,46 @@ test('scroll progress locates the visible message without scanning the full sess
   assert.ok(layoutReads < 20, `expected logarithmic layout reads, got ${layoutReads}`);
 });
 
-test('SessionDetail integrates view-state capture and restore into live reloads', () => {
+test('SessionDetail isolates unchanged rows and gives tail appends a scan-free path', () => {
   const source = readFileSync(new URL('../app/src/renderer/src/views/SessionDetail.vue', import.meta.url), 'utf8');
+  const dataSource = readFileSync(new URL('../app/src/renderer/src/data.js', import.meta.url), 'utf8');
+  const loadMessages = functionSource(source, 'loadMessages');
+  const getSkillMd = functionSource(source, 'getSkillMd');
 
+  assert.match(source, /import\s*\{[^}]*shallowRef[^}]*\}\s*from ['"]vue['"]/s);
+  assert.match(source, /const messages = shallowRef\(\[\]\)/);
+  assert.match(source, /applySnapshot/);
+  assert.match(source, /isFollowingSessionTail/);
   assert.match(source, /captureSessionViewState/);
-  assert.match(source, /reconcileSessionMessages/);
   assert.match(source, /restoreSessionViewState/);
+  assert.match(source, /:key="msg\.uuid"\s+v-memo=/);
+  assert.match(source, /v-memo="\[msg, state\.query\]"/);
   assert.match(source, /loading\.value\s*=\s*!hadContent/);
   assert.match(source, /scrollRevision/);
-  assert.match(source, /restoreScroll:\s*scrollRevision\s*===\s*scrollRevisionAtLoad/);
+  assert.match(source, /restoreScroll:\s*scrollRevision\s*===\s*scrollRevisionBeforePatch/);
   assert.match(source, /requestAnimationFrame/);
   assert.match(source, /findLastMessageAtOrAbove/);
+  assert.match(loadMessages, /if \(hadContent && !reconciliation\.tailOnly\)/);
+  assert.match(loadMessages, /if \(!reconciliation\.changed\)/);
+  assert.ok(
+    loadMessages.indexOf('applySnapshot(') < loadMessages.indexOf('captureSessionViewState('),
+    'the expensive disclosure and anchor scan happens only after classifying the snapshot',
+  );
+  assert.ok(
+    loadMessages.indexOf('applySnapshot(') < loadMessages.indexOf('isFollowingSessionTail('),
+    'tail state is sampled immediately before the patch, after the async snapshot load',
+  );
+  assert.ok(
+    loadMessages.indexOf('isFollowingSessionTail(') < loadMessages.indexOf('messages.value = reconciliation.messages'),
+    'tail state is sampled before assigning the new timeline',
+  );
+  assert.ok(
+    loadMessages.indexOf('if (!reconciliation.changed)') < loadMessages.indexOf('await nextTick()'),
+    'a no-op snapshot returns before awaiting a timeline patch',
+  );
+  assert.doesNotMatch(getSkillMd, /messages\.value/);
+  assert.match(source, /getSkillMd\(msg\)/);
+  assert.match(dataSource, /messages:\s*markRaw\(assembledMessages\)/);
 });
 
 test('live totals and scroll position remain isolated across interleaved updates', () => {