diff --git a/renderer/renderer.js b/renderer/renderer.js
index 5ffcab5..b5e0590 100644
--- a/renderer/renderer.js
+++ b/renderer/renderer.js
@@ -56,6 +56,7 @@
   let tabs = [];       // Array of tab objects
   let activeTabId = null;
   let tabIdCounter = 0;
+  let mruTabStack = []; // Most-recently-used tab IDs for Ctrl+Tab
 
   // ===== Other State =====
   let viewMode = 'split';
@@ -69,6 +70,35 @@
   let searchCaseSensitive = false;
   let searchUseRegex = false;
 
+  // ===== Character Width Measurement (#1) =====
+  const _charWidthCanvas = document.createElement('canvas');
+  const _charWidthCtx = _charWidthCanvas.getContext('2d');
+  let _cachedCharWidth = null;
+  function getCharWidth() {
+    if (_cachedCharWidth !== null) return _cachedCharWidth;
+    const style = getComputedStyle(editor);
+    _charWidthCtx.font = `${style.fontSize} ${style.fontFamily}`;
+    _cachedCharWidth = _charWidthCtx.measureText('M').width;
+    return _cachedCharWidth;
+  }
+  // Recalculate on font/theme change
+  new MutationObserver(() => { _cachedCharWidth = null; }).observe(document.documentElement, { attributes: true, attributeFilter: ['class'] });
+
+  // ===== Toast Notification (#5) =====
+  function showToast(message, duration) {
+    duration = duration || 3000;
+    let toast = document.getElementById('toast-notification');
+    if (!toast) {
+      toast = document.createElement('div');
+      toast.id = 'toast-notification';
+      document.getElementById('app').appendChild(toast);
+    }
+    toast.textContent = message;
+    toast.classList.add('show');
+    clearTimeout(toast._timer);
+    toast._timer = setTimeout(() => toast.classList.remove('show'), duration);
+  }
+
   // ===== Tab Object =====
   function createTab(filePath, content) {
     const id = ++tabIdCounter;
@@ -146,6 +176,31 @@
         const langClass = lang ? ` language-${lang}` : '';
         return `<pre><code class="hljs${langClass}">${highlighted}</code></pre>`;
       };
+      // 处理相对路径图片 → file:// URL
+      renderer.image = function (hrefOrToken, title, text) {
+        const href = (typeof hrefOrToken === 'object' && hrefOrToken !== null)
+          ? hrefOrToken.href
+          : hrefOrToken;
+        const alt = (typeof hrefOrToken === 'object' && hrefOrToken !== null)
+          ? hrefOrToken.text
+          : text;
+        const titleAttr = (typeof hrefOrToken === 'object' && hrefOrToken !== null)
+          ? hrefOrToken.title
+          : title;
+        let src = href;
+        // 相对路径转绝对路径
+        if (src && !src.startsWith('http://') && !src.startsWith('https://') && !src.startsWith('data:') && !src.startsWith('file://')) {
+          const tab = getActiveTab();
+          if (tab && tab.filePath) {
+            const dir = tab.filePath.replace(/[/\\][^/\\]+$/, '');
+            // 简单路径拼接（不依赖 require('path')）
+            const sep = dir.includes('\\') ? '\\' : '/';
+            src = 'file://' + (dir + sep + src).replace(/\\/g, '/');
+          }
+        }
+        const titleStr = titleAttr ? ` title="${titleAttr}"` : '';
+        return `<img src="${src}" alt="${alt}"${titleStr}>`;
+      };
       marked.setOptions({ gfm: true, breaks: true, pedantic: false });
       marked.use({ renderer });
     }
@@ -411,13 +466,14 @@
     }
 
     // 去重并排序
-    const uniqueStarts = [...new Set(blockStartLines)].sort((a, b) => a - b);
+    const uniqueStartsArr = [...new Set(blockStartLines)].sort((a, b) => a - b);
+    const uniqueStartsSet = new Set(uniqueStartsArr);
 
     // 将编辑器行号映射到预览 DOM 位置
     // 每个块的起始行对应一个预览元素，用线性插值填充中间行
     lineToPreviewMap = new Float32Array(totalLines);
 
-    if (uniqueStarts.length === 0) {
+    if (uniqueStartsArr.length === 0) {
       // 没有找到块边界，回退到线性映射
       for (let i = 0; i < totalLines; i++) {
         lineToPreviewMap[i] = (i / Math.max(1, totalLines - 1)) * previewPositions[previewPositions.length - 1];
@@ -427,23 +483,23 @@
 
     // 将块起始行映射到预览 DOM 索引
     const domCount = previewPositions.length;
-    for (let i = 0; i < uniqueStarts.length; i++) {
-      const lineIdx = uniqueStarts[i];
-      const domIdx = Math.min(Math.floor((i / uniqueStarts.length) * domCount), domCount - 1);
+    for (let i = 0; i < uniqueStartsArr.length; i++) {
+      const lineIdx = uniqueStartsArr[i];
+      const domIdx = Math.min(Math.floor((i / uniqueStartsArr.length) * domCount), domCount - 1);
       lineToPreviewMap[lineIdx] = previewPositions[domIdx];
     }
 
     // 线性插值填充所有行
     for (let i = 0; i < totalLines; i++) {
-      if (lineToPreviewMap[i] !== 0 && uniqueStarts.includes(i)) continue;
+      if (lineToPreviewMap[i] !== 0 && uniqueStartsSet.has(i)) continue;
 
       // 找到前后最近的已映射行
       let prevLine = -1, nextLine = -1;
       for (let j = i - 1; j >= 0; j--) {
-        if (lineToPreviewMap[j] !== 0 || uniqueStarts.includes(j)) { prevLine = j; break; }
+        if (lineToPreviewMap[j] !== 0 || uniqueStartsSet.has(j)) { prevLine = j; break; }
       }
       for (let j = i + 1; j < totalLines; j++) {
-        if (lineToPreviewMap[j] !== 0 || uniqueStarts.includes(j)) { nextLine = j; break; }
+        if (lineToPreviewMap[j] !== 0 || uniqueStartsSet.has(j)) { nextLine = j; break; }
       }
 
       if (prevLine === -1 && nextLine === -1) {
@@ -611,6 +667,7 @@
     requestAnimationFrame(() => {
       editor.scrollTop = tab.scrollTop;
       editor.scrollLeft = tab.scrollLeft;
+      lineNumbers.scrollTop = tab.scrollTop;
       editor.setSelectionRange(tab.selectionStart, tab.selectionEnd);
       previewPanel.scrollTop = tab.previewScrollTop;
       updateCursorPosition();
@@ -660,6 +717,12 @@
     // Save current state
     saveCurrentTabState();
 
+    // MRU: record previous tab before switching
+    if (activeTabId) {
+      mruTabStack = mruTabStack.filter(id => id !== activeTabId);
+      mruTabStack.unshift(activeTabId);
+    }
+
     // Switch
     activeTabId = tabId;
     const tab = getActiveTab();
@@ -687,6 +750,8 @@
 
     // Remove tab
     tabs.splice(index, 1);
+    // Clean MRU stack
+    mruTabStack = mruTabStack.filter(id => id !== tabId);
 
     // If we closed the active tab, switch to another
     if (tabId === activeTabId) {
@@ -899,13 +964,14 @@
 
       const files = e.dataTransfer.files;
       const allowedExts = ['.md', '.markdown', '.txt'];
+      let rejected = 0;
       for (const file of files) {
         const filePath = file.path;
         const ext = filePath ? filePath.substring(filePath.lastIndexOf('.')).toLowerCase() : '';
-        if (!allowedExts.includes(ext)) continue;
+        if (!allowedExts.includes(ext)) { rejected++; continue; }
         // Electron 主进程已有文件大小检查，此处为浏览器模式兜底
         if (file.size > MAX_FILE_SIZE) {
-          alert(`"${file.name}" 过大（${formatBytes(file.size)}），暂不支持超过 20MB 的文件`);
+          showToast(`"${file.name}" 过大（${formatBytes(file.size)}），暂不支持超过 20MB 的文件`);
           continue;
         }
         if (filePath && typeof window.electronAPI !== 'undefined') {
@@ -919,6 +985,9 @@
           reader.readAsText(file);
         }
       }
+      if (rejected > 0) {
+        showToast(`仅支持 .md / .markdown / .txt 文件，已忽略 ${rejected} 个文件`);
+      }
     });
   }
 
@@ -933,15 +1002,25 @@
       if (e.ctrlKey && e.key === '3') { e.preventDefault(); setViewMode('preview'); }
       if (e.ctrlKey && e.key === 't') { e.preventDefault(); createNewTab(null, ''); }
       if (e.ctrlKey && e.key === 'w') { e.preventDefault(); if (activeTabId) closeTab(activeTabId); }
-      // Ctrl+Tab / Ctrl+Shift+Tab to cycle tabs
+      // Ctrl+Tab / Ctrl+Shift+Tab to cycle tabs (MRU order)
       if (e.ctrlKey && e.key === 'Tab') {
         e.preventDefault();
         if (tabs.length > 1) {
-          const idx = getTabIndex(activeTabId);
-          const next = e.shiftKey
-            ? (idx - 1 + tabs.length) % tabs.length
-            : (idx + 1) % tabs.length;
-          switchToTab(tabs[next].id);
+          if (e.shiftKey) {
+            // Ctrl+Shift+Tab: go back in MRU history
+            if (mruTabStack.length > 0) {
+              const targetId = mruTabStack.shift();
+              // Verify target still exists
+              if (tabs.find(t => t.id === targetId)) {
+                switchToTab(targetId);
+              }
+            }
+          } else {
+            // Ctrl+Tab: forward in MRU (just switch to next in array as fallback)
+            const idx = getTabIndex(activeTabId);
+            const next = (idx + 1) % tabs.length;
+            switchToTab(tabs[next].id);
+          }
         }
       }
       // Search & Replace
@@ -1057,6 +1136,8 @@
 
     editor.addEventListener('scroll', () => {
       syncScroll();
+      // 行号跟随编辑器滚动
+      lineNumbers.scrollTop = editor.scrollTop;
       // 大文件滚动时重新渲染虚拟行号
       if (lineCountCache > 2000) {
         renderVirtualLineNumbers(lineCountCache);
@@ -1202,7 +1283,7 @@
       const col = pos - lineStarts[line];
       return {
         top: line * lineHeight + paddingTop,
-        left: col * 8 + paddingLeft
+        left: col * getCharWidth() + paddingLeft
       };
     }
 
@@ -1289,6 +1370,34 @@
   function initSidebar() {
     const settings = loadSettings();
     if (settings.sidebarCollapsed) sidebar.classList.add('collapsed');
+    if (settings.sidebarWidth) {
+      sidebar.style.width = settings.sidebarWidth + 'px';
+    }
+
+    // Sidebar resize handle (#10)
+    const handle = document.createElement('div');
+    handle.className = 'sidebar-resize-handle';
+    sidebar.appendChild(handle);
+    let isSidebarResizing = false;
+    handle.addEventListener('mousedown', (e) => {
+      isSidebarResizing = true;
+      document.body.style.cursor = 'col-resize';
+      document.body.style.userSelect = 'none';
+      e.preventDefault();
+    });
+    document.addEventListener('mousemove', (e) => {
+      if (!isSidebarResizing) return;
+      const newWidth = Math.max(180, Math.min(500, e.clientX));
+      sidebar.style.width = newWidth + 'px';
+      saveSetting('sidebarWidth', newWidth);
+    });
+    document.addEventListener('mouseup', () => {
+      if (isSidebarResizing) {
+        isSidebarResizing = false;
+        document.body.style.cursor = '';
+        document.body.style.userSelect = '';
+      }
+    });
 
     btnOpenFolder.addEventListener('click', openFolder);
     if (typeof window.electronAPI !== 'undefined') {
diff --git a/renderer/style.css b/renderer/style.css
index 20a5d46..3b60d37 100644
--- a/renderer/style.css
+++ b/renderer/style.css
@@ -750,6 +750,7 @@ html, body {
   flex-shrink: 0;
   overflow: hidden;
   user-select: none;
+  position: relative;
 }
 
 #sidebar.collapsed {
@@ -1064,3 +1065,46 @@ html, body {
 .welcome-content {
   animation: fadeIn 0.3s ease;
 }
+
+/* ===== Toast Notification ===== */
+#toast-notification {
+  position: fixed;
+  bottom: 40px;
+  left: 50%;
+  transform: translateX(-50%) translateY(20px);
+  background: var(--text);
+  color: var(--bg);
+  padding: 8px 20px;
+  border-radius: 6px;
+  font-size: 13px;
+  font-family: var(--font-ui);
+  box-shadow: var(--shadow-lg);
+  opacity: 0;
+  pointer-events: none;
+  transition: opacity 0.3s ease, transform 0.3s ease;
+  z-index: 9999;
+  max-width: 480px;
+  text-align: center;
+}
+
+#toast-notification.show {
+  opacity: 1;
+  transform: translateX(-50%) translateY(0);
+  pointer-events: auto;
+}
+
+/* ===== Sidebar Resize Handle ===== */
+.sidebar-resize-handle {
+  position: absolute;
+  top: 0;
+  right: -2px;
+  width: 4px;
+  height: 100%;
+  cursor: col-resize;
+  z-index: 5;
+}
+
+.sidebar-resize-handle:hover,
+.sidebar-resize-handle:active {
+  background: var(--primary);
+}