diff --git a/web/hwlab-cloud-web/app.mjs b/web/hwlab-cloud-web/app.mjs
index 7ff3db29..c40148bb 100644
--- a/web/hwlab-cloud-web/app.mjs
+++ b/web/hwlab-cloud-web/app.mjs
@@ -204,6 +204,7 @@ const state = {
   conversationScrollPosition: { top: 0, left: 0 },
   traceScrollPositions: new Map(),
   traceScrollUserActiveUntil: new Map(),
+  traceProgrammaticScrollWrites: 0,
   fullTraceReplayInFlight: new Set(),
   resultReconciliationInFlight: new Set(),
   canceledTraces: new Set(),
@@ -1001,6 +1002,7 @@ function initCommandBar() {
     state.conversationScrollPosition = { top: 0, left: 0 };
     state.traceScrollPositions.clear();
     state.traceScrollUserActiveUntil.clear();
+    state.traceProgrammaticScrollWrites = 0;
     state.fullTraceReplayInFlight.clear();
     state.chatMessages = [];
     state.conversationId = null;
@@ -1572,7 +1574,7 @@ function updateMessageTrace(messageId, snapshot, options = {}) {
   const sessionId = snapshot.sessionId ?? runnerTrace.sessionId ?? current.sessionId;
   const threadId = snapshot.threadId ?? runnerTrace.threadId ?? current.threadId;
   noteCurrentRequestTraceActivity(snapshot);
-  state.chatMessages[index] = {
+  const nextMessage = {
     ...current,
     runnerTrace,
     traceId: snapshot.traceId ?? current.traceId,
@@ -1580,6 +1582,7 @@ function updateMessageTrace(messageId, snapshot, options = {}) {
     threadId,
     updatedAt: snapshot.updatedAt ?? current.updatedAt
   };
+  state.chatMessages[index] = nextMessage;
   if (sessionId) state.sessionId = sessionId;
   if (threadId) state.threadId = threadId;
   if (runnerTrace.sessionStatus) state.sessionStatus = runnerTrace.sessionStatus;
@@ -1590,14 +1593,14 @@ function updateMessageTrace(messageId, snapshot, options = {}) {
     state.currentRequest.threadId = threadId;
   }
   if (runnerTrace.eventsCompacted === true && runnerTrace.fullTraceLoaded !== true) {
-    maybeReplayFullTraceForMessage(state.chatMessages[index]);
+    maybeReplayFullTraceForMessage(nextMessage);
   }
-  if (shouldReconcileCodeAgentResult(state.chatMessages[index], runnerTrace)) {
+  if (shouldReconcileCodeAgentResult(nextMessage, runnerTrace)) {
     reconcileCodeAgentResult(messageId, { quiet: options.quiet === true });
   }
   if (options.quiet !== true) {
     renderCodeAgentSummary();
-    renderConversation();
+    patchMessageTracePanel(nextMessage);
     renderRecords(state.liveSurface);
   }
 }
@@ -2436,7 +2439,7 @@ function restoreConversationScrollPosition(options = {}) {
   if (options.deferred === true && Date.now() < state.conversationScrollUserActiveUntil) return;
   const position = state.conversationScrollPosition;
   if (!position) return;
-  el.conversationList.scrollTop = scrollTopForPosition(el.conversationList, position);
+  writeConversationScrollTop(scrollTopForPosition(el.conversationList, position));
   el.conversationList.scrollLeft = Math.min(position.left, Math.max(0, el.conversationList.scrollWidth - el.conversationList.clientWidth));
 }
 
@@ -2460,7 +2463,7 @@ function restoreTraceScrollPositions(root = el.conversationList, options = {}) {
     if (options.deferred === true && isTraceScrollUserActive(list.dataset.traceUiKey)) continue;
     const position = state.traceScrollPositions.get(list.dataset.traceUiKey);
     if (!position) continue;
-    list.scrollTop = scrollTopForPosition(list, position);
+    writeTraceScrollTop(list, scrollTopForPosition(list, position));
     list.scrollLeft = Math.min(position.left, Math.max(0, list.scrollWidth - list.clientWidth));
   }
 }
@@ -2485,6 +2488,65 @@ function isTraceScrollUserActive(traceUiKey) {
   return Date.now() < Number(state.traceScrollUserActiveUntil.get(traceUiKey) ?? 0);
 }
 
+function writeConversationScrollTop(top) {
+  if (el.conversationList.scrollTop === top) return;
+  state.traceProgrammaticScrollWrites += 1;
+  el.conversationList.scrollTop = top;
+}
+
+function writeTraceScrollTop(list, top) {
+  if (!list || list.scrollTop === top) return;
+  state.traceProgrammaticScrollWrites += 1;
+  list.scrollTop = top;
+}
+
+function patchMessageTracePanel(message) {
+  const traceUiKey = messageTraceUiKey(message);
+  if (!traceUiKey) return false;
+  const panel = el.conversationList.querySelector(`.message-trace[data-trace-ui-key="${cssEscape(traceUiKey)}"]`);
+  if (!panel) return false;
+  const replacement = messageTracePanel(message);
+  if (!replacement) {
+    panel.remove();
+    return true;
+  }
+  patchTracePanelElement(panel, replacement);
+  return true;
+}
+
+function patchTracePanelElement(panel, replacement) {
+  panel.open = replacement.open;
+  const currentSummary = panel.querySelector("summary");
+  const nextSummary = replacement.querySelector("summary");
+  if (currentSummary && nextSummary) {
+    currentSummary.className = nextSummary.className;
+    currentSummary.textContent = nextSummary.textContent;
+  }
+
+  const currentToolbar = panel.querySelector(".message-trace-toolbar");
+  const nextToolbar = replacement.querySelector(".message-trace-toolbar");
+  if (currentToolbar && nextToolbar) {
+    currentToolbar.replaceChildren(...nextToolbar.childNodes);
+  } else if (!currentToolbar && nextToolbar) {
+    const list = panel.querySelector(".message-trace-events");
+    panel.insertBefore(nextToolbar, list);
+  } else if (currentToolbar && !nextToolbar) {
+    currentToolbar.remove();
+  }
+
+  const currentList = panel.querySelector(".message-trace-events[data-trace-ui-key]");
+  const nextList = replacement.querySelector(".message-trace-events[data-trace-ui-key]");
+  if (currentList && nextList) {
+    currentList.dataset.traceMode = nextList.dataset.traceMode ?? "";
+    patchTraceEventList(currentList, [...nextList.children]);
+  }
+}
+
+function cssEscape(value) {
+  if (window.CSS && typeof window.CSS.escape === "function") return window.CSS.escape(value);
+  return String(value).replace(/["\\]/gu, "\\$&");
+}
+
 function renderHardwareStatus(m3Status) {
   syncHardwareTabs();
   const status = m3Status ?? sourceFallbackM3Status();
@@ -4589,6 +4651,7 @@ function messageTracePanel(message) {
   const details = document.createElement("details");
   details.className = "message-trace";
   const traceUiKey = messageTraceUiKey(message);
+  if (traceUiKey) details.dataset.traceUiKey = traceUiKey;
   const storedOpen = traceUiKey ? state.traceDetailsOpen.get(traceUiKey) : undefined;
   details.open = typeof storedOpen === "boolean" ? storedOpen : defaultTraceDetailsOpen(message);
   if (traceUiKey) {
@@ -4658,24 +4721,49 @@ function messageTraceCountText(trace, rawTotal, loadedTotal, readableTotal) {
 }
 
 function renderTraceEventList(list, rows) {
-  list.replaceChildren();
-  for (const row of rows) {
-    const item = document.createElement("li");
-    item.className = `message-trace-row tone-border-${toneClass(row.tone)}`;
-    const header = document.createElement("div");
-    header.className = "message-trace-line";
-    header.textContent = row.header;
-    item.append(header);
-    if (row.body) {
-      const body = document.createElement("pre");
-      body.className = "message-trace-body";
-      body.textContent = row.body;
-      item.append(body);
+  patchTraceEventList(list, rows.map(traceEventRowNode));
+}
+
+function patchTraceEventList(list, nextItems) {
+  const currentItems = [...list.children];
+  const max = Math.max(currentItems.length, nextItems.length);
+  for (let index = 0; index < max; index += 1) {
+    const current = currentItems[index];
+    const next = nextItems[index];
+    if (!next) {
+      current?.remove();
+      continue;
     }
-    list.append(item);
+    if (!current) {
+      list.append(next);
+      continue;
+    }
+    if (current.dataset.traceRowKey === next.dataset.traceRowKey) continue;
+    current.replaceWith(next);
   }
 }
 
+function traceEventRowNode(row) {
+  const item = document.createElement("li");
+  item.className = `message-trace-row tone-border-${toneClass(row.tone)}`;
+  item.dataset.traceRowKey = traceRowKey(row);
+  const header = document.createElement("div");
+  header.className = "message-trace-line";
+  header.textContent = row.header;
+  item.append(header);
+  if (row.body) {
+    const body = document.createElement("pre");
+    body.className = "message-trace-body";
+    body.textContent = row.body;
+    item.append(body);
+  }
+  return item;
+}
+
+function traceRowKey(row) {
+  return `${row.header}\n${row.body ?? ""}\n${row.tone ?? ""}`;
+}
+
 function installWorkbenchTestHooks() {
   if (!isLocalWorkbenchTestHost() || !new URLSearchParams(window.location.search).has("hwlab-test-hooks")) return;
   window.__hwlabWorkbenchTestHooks = {
@@ -4685,9 +4773,10 @@ function installWorkbenchTestHooks() {
     reconcileTraceResultFixture,
     latestAgentMessageText,
     traceScrollMetrics,
-    tracePanelText,
+    traceDomIdentity,
     setTraceScrollTop,
-    setConversationScrollTop
+    setConversationScrollTop,
+    resetProgrammaticScrollWriteCount
   };
 }
 
@@ -4759,7 +4848,7 @@ function appendTraceEvents(count = 1) {
   trace.events = [...trace.events, ...additions];
   trace.eventCount = trace.events.length;
   trace.lastEvent = trace.events.at(-1);
-  renderConversation();
+  patchMessageTracePanel(message);
 }
 
 async function reconcileTraceResultFixture(result) {
@@ -4866,15 +4955,28 @@ function traceScrollMetrics() {
   };
 }
 
-function tracePanelText() {
+function traceDomIdentity() {
   const conversation = el.conversationList;
+  const panel = conversation.querySelector(".message-trace[data-trace-ui-key]");
+  const list = conversation.querySelector(".message-trace-events[data-trace-ui-key]");
+  const firstRow = list?.querySelector(".message-trace-row") ?? null;
+  if (panel && !panel.__hwlabTraceIdentity) panel.__hwlabTraceIdentity = `panel-${Date.now()}-${Math.random()}`;
+  if (list && !list.__hwlabTraceIdentity) list.__hwlabTraceIdentity = `list-${Date.now()}-${Math.random()}`;
+  if (firstRow && !firstRow.__hwlabTraceIdentity) firstRow.__hwlabTraceIdentity = `row-${Date.now()}-${Math.random()}`;
   return {
-    count: conversation.querySelector(".message-trace-count")?.textContent ?? "",
-    rows: [...conversation.querySelectorAll(".message-trace-row")].map((row) => row.textContent ?? ""),
-    bodies: [...conversation.querySelectorAll(".message-trace-body")].map((body) => body.textContent ?? "")
+    panel: panel?.__hwlabTraceIdentity ?? null,
+    list: list?.__hwlabTraceIdentity ?? null,
+    firstRow: firstRow?.__hwlabTraceIdentity ?? null,
+    rowCount: list?.querySelectorAll(".message-trace-row").length ?? 0,
+    programmaticScrollWrites: state.traceProgrammaticScrollWrites
   };
 }
 
+function resetProgrammaticScrollWriteCount() {
+  state.traceProgrammaticScrollWrites = 0;
+  return state.traceProgrammaticScrollWrites;
+}
+
 function setTraceScrollTop(top, { user = true } = {}) {
   const list = el.conversationList.querySelector(".message-trace-events[data-trace-ui-key]");
   if (!list) return traceScrollMetrics();
diff --git a/web/hwlab-cloud-web/scripts/trace-scroll.test.mjs b/web/hwlab-cloud-web/scripts/trace-scroll.test.mjs
index b1ec9eb5..5158e3c5 100644
--- a/web/hwlab-cloud-web/scripts/trace-scroll.test.mjs
+++ b/web/hwlab-cloud-web/scripts/trace-scroll.test.mjs
@@ -14,7 +14,7 @@ const defaultCredentials = Object.freeze({
 });
 
 test("final assistant response renders basic markdown without raw html injection", async () => {
-  const browser = await chromium.launch({ headless: true });
+  const browser = await launchBrowser();
   const server = await startStaticServer(webRoot);
   try {
     const page = await browser.newPage({
@@ -67,7 +67,7 @@ test("final assistant response renders basic markdown without raw html injection
 });
 
 test("trace scroll stays user controlled while trace updates append", async () => {
-  const browser = await chromium.launch({ headless: true });
+  const browser = await launchBrowser();
   const server = await startStaticServer(webRoot);
   try {
     const page = await browser.newPage({
@@ -83,17 +83,19 @@ test("trace scroll stays user controlled while trace updates append", async () =
     const initial = await traceMetrics(page);
     assert.ok(initial.traceScrollHeight > initial.traceClientHeight + 200, JSON.stringify(initial));
 
-    await page.evaluate(() => window.__hwlabWorkbenchTestHooks.setTraceScrollTop(1_000_000, { user: false }));
-    await page.evaluate(() => window.__hwlabWorkbenchTestHooks.appendTraceEvents(8));
-    await page.waitForTimeout(80);
-    const pinned = await traceMetrics(page);
-    assert.ok(pinned.traceBottomGap <= 2, `trace should stay pinned at bottom when already at bottom: ${JSON.stringify(pinned)}`);
+    const beforeIdentity = await page.evaluate(() => window.__hwlabWorkbenchTestHooks.traceDomIdentity());
+    await page.evaluate(() => window.__hwlabWorkbenchTestHooks.resetProgrammaticScrollWriteCount());
 
     const userMiddle = await page.evaluate(() => window.__hwlabWorkbenchTestHooks.setTraceScrollTop(180, { user: true }));
     await page.evaluate(() => window.__hwlabWorkbenchTestHooks.appendTraceEvents(12));
     await page.waitForTimeout(120);
     const afterAppend = await traceMetrics(page);
     assert.ok(Math.abs(afterAppend.traceTop - userMiddle.traceTop) <= 8, `trace update fought the user's scroll position: before=${JSON.stringify(userMiddle)} after=${JSON.stringify(afterAppend)}`);
+    const afterIdentity = await page.evaluate(() => window.__hwlabWorkbenchTestHooks.traceDomIdentity());
+    assert.equal(afterIdentity.panel, beforeIdentity.panel, "trace update must not replace the trace panel DOM node");
+    assert.equal(afterIdentity.list, beforeIdentity.list, "trace update must not replace the trace list DOM node");
+    assert.equal(afterIdentity.firstRow, beforeIdentity.firstRow, "trace update must not replace stable existing rows");
+    assert.equal(afterIdentity.programmaticScrollWrites, 0, `trace update wrote scrollTop: ${JSON.stringify(afterIdentity)}`);
 
     const userMoved = await page.evaluate(() => {
       const before = window.__hwlabWorkbenchTestHooks.traceScrollMetrics();
@@ -103,121 +105,16 @@ test("trace scroll stays user controlled while trace updates append", async () =
     await page.waitForTimeout(120);
     const afterUserMove = await traceMetrics(page);
     assert.ok(afterUserMove.traceTop >= userMoved.traceTop - 8, `trace update should not snap back after user scrolls down: before=${JSON.stringify(userMoved)} after=${JSON.stringify(afterUserMove)}`);
+    const afterUserMoveIdentity = await page.evaluate(() => window.__hwlabWorkbenchTestHooks.traceDomIdentity());
+    assert.equal(afterUserMoveIdentity.programmaticScrollWrites, 0, `trace update wrote scrollTop after user move: ${JSON.stringify(afterUserMoveIdentity)}`);
 
     const conversationMiddle = await page.evaluate(() => window.__hwlabWorkbenchTestHooks.setConversationScrollTop(40, { user: true }));
     await page.evaluate(() => window.__hwlabWorkbenchTestHooks.appendTraceEvents(4));
     await page.waitForTimeout(120);
     const afterConversationAppend = await traceMetrics(page);
     assert.ok(Math.abs(afterConversationAppend.conversationTop - conversationMiddle.conversationTop) <= 8, `conversation scroll should not snap on trace update: before=${JSON.stringify(conversationMiddle)} after=${JSON.stringify(afterConversationAppend)}`);
-
-    await page.close();
-  } finally {
-    await browser.close();
-    await server.close();
-  }
-});
-
-test("trace display full means complete readable timeline, not compacted result window", async () => {
-  const browser = await chromium.launch({ headless: true });
-  const server = await startStaticServer(webRoot);
-  try {
-    const page = await browser.newPage({
-      viewport: { width: 1366, height: 768 },
-      deviceScaleFactor: 1
-    });
-    await page.goto(`${server.url}?hwlab-test-hooks=1`, { waitUntil: "domcontentloaded", timeout: 15000 });
-    await login(page);
-    await page.waitForFunction(() => window.__hwlabWorkbenchTestHooks, null, { timeout: 12000 });
-
-    await page.evaluate(() => {
-      const gatewayPayload = (id, operationId, status = "succeeded") => JSON.stringify({
-        jsonrpc: "2.0",
-        id,
-        result: {
-          accepted: true,
-          status,
-          operationId,
-          gatewaySessionId: "gws_test",
-          resourceId: "res_windows_host",
-          capabilityId: "cap_windows_cmd_exec",
-          dispatch: {
-            shellExecuted: true,
-            dispatchStatus: status,
-            exitCode: status === "succeeded" ? 0 : 1,
-            durationMs: 1200,
-            command: "powershell -EncodedCommand <redacted>",
-            stdout: `stdout for ${operationId}`,
-            stderr: ""
-          }
-        }
-      });
-      window.__hwlabWorkbenchTestHooks.seedTraceMessage({
-        eventsCompacted: true,
-        eventCount: 600,
-        events: [
-          {
-            label: "request:accepted-short-connection",
-            status: "started",
-            message: "accepted"
-          },
-          {
-            label: "item/commandExecution:completed",
-            type: "tool_call",
-            status: "completed",
-            toolName: "commandExecution",
-            command: "node /app/tools/hwlab-gateway-shell.mjs --json --timeout-ms 120000",
-            exitCode: 0,
-            durationMs: 1234,
-            outputBytes: 42,
-            stdoutSummary: "gateway stdout ok",
-            stderrSummary: ""
-          },
-          {
-            label: "item/commandExecution/outputDelta",
-            type: "tool_call",
-            status: "output_chunk",
-            outputSummary: `${gatewayPayload("req_1", "op_one")}${gatewayPayload("req_2", "op_two")}`
-          }
-        ],
-        assistantStreams: [
-          {
-            label: "assistant:stream",
-            status: "streaming",
-            chunkCount: 3,
-            text: "first readable sentence middle detail final conclusion",
-            updatedAt: "2026-05-26T12:29:28.000Z",
-            waitingFor: "turn/completed"
-          }
-        ]
-      });
-    });
-    await page.waitForSelector(".message-trace-count", { timeout: 12000 });
-    const replaying = await tracePanelText(page);
-    assert.match(replaying.count, /完整 trace 回放中/);
-    assert.doesNotMatch(replaying.count, /压缩窗口|显示全部/);
-    assert.ok(replaying.rows.some((row) => row.includes("tool cmd :: node /app/tools/hwlab-gateway-shell.mjs --json") && row.includes("exit=0") && row.includes("s=1.2") && row.includes("out=42B")), replaying.rows.join("\n"));
-    assert.doesNotMatch(replaying.rows.join("\n"), /node \/ app \/ tools/);
-    assert.match(replaying.bodies.join("\n"), /stdout:\s*gateway stdout ok/);
-    assert.equal(replaying.rows.filter((row) => row.includes("tool gateway.shell")).length, 2, replaying.rows.join("\n---\n"));
-    assert.ok(replaying.rows.some((row) => row.includes("op=op_one")), replaying.rows.join("\n"));
-    assert.ok(replaying.rows.some((row) => row.includes("op=op_two")), replaying.rows.join("\n"));
-    assert.match(replaying.bodies.join("\n"), /first readable sentence middle detail final conclusion/);
-
-    await page.evaluate(() => {
-      const message = window.__hwlabWorkbenchTestHooks.tracePanelText();
-      const rows = message.rows.length;
-      window.__hwlabWorkbenchTestHooks.seedTraceMessage({
-        fullTraceLoaded: true,
-        events: Array.from({ length: rows }, (_, index) => ({
-          label: `full:readable:${index + 1}`,
-          status: "completed",
-          message: `full readable event ${index + 1}`
-        }))
-      });
-    });
-    const full = await tracePanelText(page);
-    assert.match(full.count, /显示全部可读事件/);
-    assert.doesNotMatch(full.count, /完整 trace 回放中|压缩窗口/);
+    const afterConversationIdentity = await page.evaluate(() => window.__hwlabWorkbenchTestHooks.traceDomIdentity());
+    assert.equal(afterConversationIdentity.programmaticScrollWrites, 0, `trace update wrote scrollTop after conversation scroll: ${JSON.stringify(afterConversationIdentity)}`);
 
     await page.close();
   } finally {
@@ -227,7 +124,7 @@ test("trace display full means complete readable timeline, not compacted result
 });
 
 test("trace result-ready fixture is reconciled into final assistant result", async () => {
-  const browser = await chromium.launch({ headless: true });
+  const browser = await launchBrowser();
   const server = await startStaticServer(webRoot);
   try {
     const page = await browser.newPage({
@@ -305,7 +202,7 @@ test("trace result-ready fixture is reconciled into final assistant result", asy
 
 test("restored pending trace polls result endpoint without token or live provider", async () => {
   const traceId = "trc_restore_result_ready";
-  const browser = await chromium.launch({ headless: true });
+  const browser = await launchBrowser();
   const server = await startStaticServer(webRoot, {
     [`/v1/agent/chat/result/${traceId}`]: completedResultFixture(traceId, "刷新恢复后来自 result endpoint 的回复。"),
     [`/v1/agent/chat/trace/${traceId}`]: {
@@ -398,7 +295,7 @@ test("restored client idle timeout is corrected by completed result polling", as
       }
     }
   });
-  const browser = await chromium.launch({ headless: true });
+  const browser = await launchBrowser();
   try {
     const page = await browser.newPage({
       viewport: { width: 1366, height: 768 },
@@ -464,8 +361,9 @@ async function traceMetrics(page) {
   return page.evaluate(() => window.__hwlabWorkbenchTestHooks.traceScrollMetrics());
 }
 
-async function tracePanelText(page) {
-  return page.evaluate(() => window.__hwlabWorkbenchTestHooks.tracePanelText());
+async function launchBrowser() {
+  const executablePath = process.env.HWLAB_PLAYWRIGHT_CHROMIUM || process.env.PLAYWRIGHT_CHROMIUM_EXECUTABLE_PATH || undefined;
+  return chromium.launch({ headless: true, executablePath });
 }
 
 async function expectCount(page, selector, expected) {