diff --git a/main.js b/main.js
index b2287ae..c7725fa 100644
--- a/main.js
+++ b/main.js
@@ -195,16 +195,15 @@ function switchActiveFile(filePath) {
 // IPC Handlers
 
 // Save file to a specific path with watcher management
-function saveToPath(filePath, content) {
+async function saveToPath(filePath, content) {
   stopWatching();
   try {
     isSelfWriting = true;
-    fs.writeFileSync(filePath, content, 'utf-8');
-    // 写入后短暂延迟再恢复监听，让 fs.watch 错过自身写入的事件
+    await fs.promises.writeFile(filePath, content, 'utf-8');
     isSelfWriting = false;
   } catch (err) {
     isSelfWriting = false;
-    startWatching(activeFilePath); // Restore watcher on failure
+    startWatching(activeFilePath);
     return { success: false, error: err.message };
   }
   activeFilePath = filePath;
@@ -250,7 +249,7 @@ ipcMain.handle('file:read', async (event, filePath) => {
 ipcMain.handle('file:save', async (event, { filePath, content }) => {
   try {
     if (filePath) {
-      return saveToPath(filePath, content);
+      return await saveToPath(filePath, content);
     } else {
       const result = await dialog.showSaveDialog(mainWindow, {
         filters: [
@@ -258,7 +257,7 @@ ipcMain.handle('file:save', async (event, { filePath, content }) => {
         ]
       });
       if (!result.canceled) {
-        return saveToPath(result.filePath, content);
+        return await saveToPath(result.filePath, content);
       }
       return { success: false, canceled: true };
     }
@@ -275,7 +274,7 @@ ipcMain.handle('file:saveAs', async (event, { content }) => {
       ]
     });
     if (!result.canceled) {
-      return saveToPath(result.filePath, content);
+      return await saveToPath(result.filePath, content);
     }
     return { success: false, canceled: true };
   } catch (err) {
diff --git a/package.json b/package.json
index 98ec914..3129efe 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "marklite",
-  "version": "1.2.0",
+  "version": "1.2.1",
   "description": "Lightweight Markdown Reader for Windows",
   "main": "main.js",
   "scripts": {
diff --git a/renderer/index.html b/renderer/index.html
index 9913871..e6e7b71 100644
--- a/renderer/index.html
+++ b/renderer/index.html
@@ -112,6 +112,7 @@
         <!-- Search & Replace Bar -->
         <div id="search-bar" class="hidden">
           <div class="search-row">
+            <button id="btn-toggle-replace" class="search-opt-btn" title="展开替换行">▾</button>
             <input type="text" id="search-input" placeholder="查找..." />
             <span id="search-count"></span>
             <button id="btn-case" class="search-opt-btn" title="区分大小写 (Alt+C)">Aa</button>
diff --git a/renderer/renderer.js b/renderer/renderer.js
index b5e0590..9df60d0 100644
--- a/renderer/renderer.js
+++ b/renderer/renderer.js
@@ -38,6 +38,7 @@
   const btnSearchClose = document.getElementById('btn-search-close');
   const btnReplace = document.getElementById('btn-replace');
   const btnReplaceAll = document.getElementById('btn-replace-all');
+  const btnToggleReplace = document.getElementById('btn-toggle-replace');
 
   // Buttons
   const btnOpen = document.getElementById('btn-open');
@@ -69,6 +70,7 @@
   let searchIndex = -1;
   let searchCaseSensitive = false;
   let searchUseRegex = false;
+  let searchHighlightsOverlay = null; // 当前搜索高亮 overlay 元素引用
 
   // ===== Character Width Measurement (#1) =====
   const _charWidthCanvas = document.createElement('canvas');
@@ -84,6 +86,27 @@
   // Recalculate on font/theme change
   new MutationObserver(() => { _cachedCharWidth = null; }).observe(document.documentElement, { attributes: true, attributeFilter: ['class'] });
 
+  // ===== Actual Line Height Measurement =====
+  // textarea 的实际渲染行高可能与 CSS line-height 计算值有微小差异，
+  // 导致行号面板与编辑器内容逐渐错位。动态测量真实行高来解决。
+  let _cachedLineHeight = null;
+  function getLineHeight() {
+    if (_cachedLineHeight !== null) return _cachedLineHeight;
+    const style = getComputedStyle(editor);
+    const measure = document.createElement('div');
+    measure.style.cssText = `position:absolute;visibility:hidden;font-family:${style.fontFamily};font-size:${style.fontSize};line-height:${style.lineHeight};white-space:pre;width:0;`;
+    measure.textContent = 'X';
+    document.body.appendChild(measure);
+    const singleHeight = measure.offsetHeight;
+    measure.textContent = 'X\nX';
+    const doubleHeight = measure.offsetHeight;
+    document.body.removeChild(measure);
+    _cachedLineHeight = doubleHeight - singleHeight || 20.8;
+    return _cachedLineHeight;
+  }
+  // 字体/主题变化时清除缓存
+  new MutationObserver(() => { _cachedLineHeight = null; _cachedCharWidth = null; }).observe(document.documentElement, { attributes: true, attributeFilter: ['class'] });
+
   // ===== Toast Notification (#5) =====
   function showToast(message, duration) {
     duration = duration || 3000;
@@ -348,7 +371,7 @@
       // 小文件：一次性生成全部行号（使用数组 join 比字符串拼接快）
       const arr = new Array(count);
       for (let i = 0; i < count; i++) {
-        arr[i] = '<div class="line-num">' + (i + 1) + '</div>';
+        arr[i] = '<div class="line-num" style="height:' + getLineHeight() + 'px">' + (i + 1) + '</div>';
       }
       lineNumbers.innerHTML = arr.join('');
     }
@@ -356,7 +379,7 @@
 
   // 虚拟化行号：只渲染当前可视区域附近的行号
   function renderVirtualLineNumbers(totalLines) {
-    const lineHeight = 20.8; // 与 CSS line-height 一致
+    const lineHeight = getLineHeight();
     const scrollTop = editor.scrollTop;
     const viewportHeight = editor.clientHeight;
     const buffer = 20; // 上下缓冲行数
@@ -518,7 +541,7 @@
 
   function syncScroll() {
     if (viewMode !== 'split') return;
-    const lineHeight = parseFloat(getComputedStyle(editor).lineHeight) || 20.8;
+    const lineHeight = getLineHeight();
     const scrollTop = editor.scrollTop;
     const topLine = Math.floor(scrollTop / lineHeight);
 
@@ -547,6 +570,7 @@
       updateLineNumbers();
       updateFileSize();
       cachePreviewPositions();
+      _cachedLineHeight = null; // 内容变化后重新测量行高
     }, 150);
   }
 
@@ -1128,7 +1152,6 @@
       const tab = getActiveTab();
       if (tab) {
         tab.isModified = true;
-        tab.content = editor.value;
         setModified(true);
       }
       scheduleUpdate();
@@ -1142,6 +1165,10 @@
       if (lineCountCache > 2000) {
         renderVirtualLineNumbers(lineCountCache);
       }
+      // 搜索高亮跟随编辑器滚动更新位置
+      if (searchHighlightsOverlay) {
+        searchHighlightsOverlay.style.top = -editor.scrollTop + 'px';
+      }
     });
     editor.addEventListener('click', updateCursorPosition);
     editor.addEventListener('keyup', updateCursorPosition);
@@ -1179,6 +1206,7 @@
   function openSearch(showReplace) {
     searchBar.classList.remove('hidden');
     replaceRow.classList.toggle('hidden', !showReplace);
+    btnToggleReplace.classList.toggle('expanded', showReplace);
     searchInput.focus();
     // Pre-fill with selected text
     const selected = editor.value.substring(editor.selectionStart, editor.selectionEnd);
@@ -1190,6 +1218,8 @@
 
   function closeSearch() {
     searchBar.classList.add('hidden');
+    replaceRow.classList.add('hidden');
+    btnToggleReplace.classList.remove('expanded');
     clearHighlights();
     searchMatches = [];
     searchIndex = -1;
@@ -1258,28 +1288,43 @@
     return matches;
   }
 
+  // ===== Search Highlight Cache =====
+  let cachedLineStarts = null;
+  let cachedLineStartsContent = null;
+
+  // 二分查找：返回 pos 所在行号（O(log N) 替代原来的线性扫描 O(N)）
+  function getLineNumber(pos) {
+    if (!cachedLineStarts) return 0;
+    let lo = 0, hi = cachedLineStarts.length - 1;
+    while (lo < hi) {
+      const mid = (lo + hi + 1) >> 1;
+      if (cachedLineStarts[mid] <= pos) lo = mid; else hi = mid - 1;
+    }
+    return lo;
+  }
+
   function highlightMatches() {
     clearHighlights();
     if (searchMatches.length === 0) return;
 
     const content = editor.value;
-    const lineHeight = parseFloat(getComputedStyle(editor).lineHeight) || 20.8;
+    const lineHeight = getLineHeight();
     const editorStyle = getComputedStyle(editor);
     const paddingTop = parseFloat(editorStyle.paddingTop);
     const paddingLeft = parseFloat(editorStyle.paddingLeft);
 
-    // Compute line starts for fast lookup
-    const lineStarts = [0];
-    for (let i = 0; i < content.length; i++) {
-      if (content.charCodeAt(i) === 10) lineStarts.push(i + 1);
+    // 缓存 lineStarts：内容未变时跳过重建（O(N) → O(1)）
+    if (cachedLineStartsContent !== content) {
+      cachedLineStarts = [0];
+      for (let i = 0; i < content.length; i++) {
+        if (content.charCodeAt(i) === 10) cachedLineStarts.push(i + 1);
+      }
+      cachedLineStartsContent = content;
     }
+    const lineStarts = cachedLineStarts;
 
     function posToXY(pos) {
-      let line = 0;
-      for (let i = 0; i < lineStarts.length; i++) {
-        if (lineStarts[i] > pos) break;
-        line = i;
-      }
+      const line = getLineNumber(pos);
       const col = pos - lineStarts[line];
       return {
         top: line * lineHeight + paddingTop,
@@ -1289,10 +1334,11 @@
 
     const overlay = document.createElement('div');
     overlay.id = 'search-highlights';
-    overlay.style.cssText = 'position:absolute;top:0;left:0;right:0;bottom:0;pointer-events:none;overflow:hidden;z-index:1;';
+    overlay.style.cssText = `position:absolute;top:${-editor.scrollTop}px;left:0;right:0;height:${editor.scrollHeight}px;pointer-events:none;overflow:visible;z-index:1;`;
     const wrapper = document.getElementById('editor-wrapper');
     wrapper.style.position = 'relative';
     wrapper.appendChild(overlay);
+    searchHighlightsOverlay = overlay;
 
     for (let i = 0; i < searchMatches.length; i++) {
       const m = searchMatches[i];
@@ -1309,6 +1355,7 @@
   function clearHighlights() {
     const old = document.getElementById('search-highlights');
     if (old) old.remove();
+    searchHighlightsOverlay = null;
   }
 
   function scrollToMatch(index) {
@@ -1316,13 +1363,17 @@
     searchIndex = index;
     const m = searchMatches[index];
     editor.setSelectionRange(m.start, m.end);
-    // Scroll the match into view
-    const lineHeight = parseFloat(getComputedStyle(editor).lineHeight) || 20.8;
-    const content = editor.value;
-    let line = 0;
-    for (let i = 0; i < m.start; i++) {
-      if (content.charCodeAt(i) === 10) line++;
+    // 复用缓存的 lineStarts + 二分查找（替代逐字符 O(N) 扫描）
+    const lineHeight = getLineHeight();
+    if (!cachedLineStarts) {
+      const content = editor.value;
+      cachedLineStarts = [0];
+      for (let i = 0; i < content.length; i++) {
+        if (content.charCodeAt(i) === 10) cachedLineStarts.push(i + 1);
+      }
+      cachedLineStartsContent = content;
     }
+    const line = getLineNumber(m.start);
     const matchTop = line * lineHeight;
     const viewTop = editor.scrollTop;
     const viewBottom = viewTop + editor.clientHeight;
@@ -1357,12 +1408,20 @@
   function replaceAll() {
     if (searchMatches.length === 0) return;
     const replacement = replaceInput.value;
-    // Replace from end to start to preserve positions
+    const content = editor.value;
+    // 拼接替换后的完整内容
+    let newContent = '';
+    let lastEnd = 0;
     for (let i = searchMatches.length - 1; i >= 0; i--) {
-      const m = searchMatches[i];
-      editor.setSelectionRange(m.start, m.end);
-      document.execCommand('insertText', false, replacement);
+      newContent = content.substring(lastEnd, searchMatches[i].start) + replacement + newContent;
+      lastEnd = searchMatches[i].end;
     }
+    newContent = content.substring(0, searchMatches[0].start) + newContent + content.substring(searchMatches[searchMatches.length - 1].end);
+    // 使用 select + execCommand 实现：一次原子操作进 undo 栈
+    // Ctrl+Z 可一次性撤销全部替换
+    editor.focus();
+    editor.select();
+    document.execCommand('insertText', false, newContent);
     doSearch();
   }
 
@@ -1627,6 +1686,12 @@
   // ===== Search Bar Init =====
   function initSearchBar() {
     searchInput.addEventListener('input', doSearch);
+    btnToggleReplace.addEventListener('click', () => {
+      const isVisible = !replaceRow.classList.contains('hidden');
+      replaceRow.classList.toggle('hidden');
+      btnToggleReplace.classList.toggle('expanded', !isVisible);
+      if (!isVisible) replaceInput.focus();
+    });
     btnCase.addEventListener('click', () => toggleSearchOption('case'));
     btnRegex.addEventListener('click', () => toggleSearchOption('regex'));
     btnNext.addEventListener('click', findNext);
diff --git a/renderer/style.css b/renderer/style.css
index 3b60d37..0de275d 100644
--- a/renderer/style.css
+++ b/renderer/style.css
@@ -886,6 +886,15 @@ html, body {
 }
 
 /* ===== Search & Replace Bar ===== */
+#btn-toggle-replace {
+  font-size: 10px;
+  transition: transform 0.15s ease;
+}
+
+#btn-toggle-replace.expanded {
+  transform: rotate(90deg);
+}
+
 #search-bar {
   background: var(--search-bg);
   border-bottom: 1px solid var(--search-border);