diff --git a/assets/styles/_sidebar.css b/assets/styles/_sidebar.css
index 48286d9..f3710de 100644
--- a/assets/styles/_sidebar.css
+++ b/assets/styles/_sidebar.css
@@ -220,12 +220,27 @@
   /* 移除顶部 padding，避免标题上方出现缝隙 */
   scrollbar-width: none;
   /* Firefox 隐藏滚动条 */
+  opacity: 0;
+  visibility: hidden;
+  pointer-events: none;
+  transition:
+    opacity 0.18s ease,
+    visibility 0s linear 0.18s;
 }
 
 .sidebar-submenu-panel:empty {
   display: none;
 }
 
+.sidebar.submenu-panel-visible .sidebar-submenu-panel:not(:empty) {
+  opacity: 1;
+  visibility: visible;
+  pointer-events: auto;
+  transition:
+    opacity 0.18s ease,
+    visibility 0s linear 0s;
+}
+
 .sidebar-submenu-panel::-webkit-scrollbar {
   display: none;
   /* Webkit 浏览器隐藏滚动条 */
diff --git a/src/runtime/app/ui.js b/src/runtime/app/ui.js
index ffa4dfb..8203ed3 100644
--- a/src/runtime/app/ui.js
+++ b/src/runtime/app/ui.js
@@ -13,6 +13,10 @@ module.exports = function initUi(state, dom) {
     themeIcon,
   } = dom;
 
+  const SUBMENU_PANEL_VISIBLE_CLASS = 'submenu-panel-visible';
+  const SIDEBAR_LAYOUT_TRANSITION_MS = 300;
+  let submenuPanelRevealTimer = null;
+
   // 移除预加载类，允许 CSS 过渡效果
   document.documentElement.classList.remove('preload');
 
@@ -100,6 +104,32 @@ module.exports = function initUi(state, dom) {
     return window.innerWidth <= 768;
   }
 
+  function clearSubmenuPanelRevealTimer() {
+    if (submenuPanelRevealTimer) {
+      window.clearTimeout(submenuPanelRevealTimer);
+      submenuPanelRevealTimer = null;
+    }
+  }
+
+  function hideSubmenuPanelImmediately() {
+    clearSubmenuPanelRevealTimer();
+    sidebar.classList.remove(SUBMENU_PANEL_VISIBLE_CLASS);
+  }
+
+  function showSubmenuPanelImmediately() {
+    clearSubmenuPanelRevealTimer();
+    sidebar.classList.add(SUBMENU_PANEL_VISIBLE_CLASS);
+  }
+
+  function revealSubmenuPanelAfterSidebarTransition() {
+    clearSubmenuPanelRevealTimer();
+    submenuPanelRevealTimer = window.setTimeout(() => {
+      if (!state.isSidebarCollapsed && !sidebar.classList.contains('collapsed')) {
+        sidebar.classList.add(SUBMENU_PANEL_VISIBLE_CLASS);
+      }
+    }, SIDEBAR_LAYOUT_TRANSITION_MS);
+  }
+
   // 侧边栏折叠功能
   function toggleSidebarCollapse() {
     // 仅在交互时启用布局相关动画，避免首屏闪烁
@@ -107,6 +137,11 @@ module.exports = function initUi(state, dom) {
 
     state.isSidebarCollapsed = !state.isSidebarCollapsed;
 
+    if (state.isSidebarCollapsed) {
+      // 收起时立即隐藏目录面板，避免在动画过程中残留。
+      hideSubmenuPanelImmediately();
+    }
+
     // 使用 requestAnimationFrame 确保平滑过渡
     requestAnimationFrame(() => {
       sidebar.classList.toggle('collapsed', state.isSidebarCollapsed);
@@ -114,6 +149,11 @@ module.exports = function initUi(state, dom) {
 
       // 保存折叠状态到 localStorage
       localStorage.setItem('sidebarCollapsed', state.isSidebarCollapsed ? 'true' : 'false');
+
+      if (!state.isSidebarCollapsed) {
+        // 展开后再淡入目录面板，避免和宽度动画抢节奏。
+        revealSubmenuPanelAfterSidebarTransition();
+      }
     });
   }
 
@@ -125,8 +165,10 @@ module.exports = function initUi(state, dom) {
     // 图标状态与折叠状态保持一致
     if (savedState === 'true' && !isMobile()) {
       state.isSidebarCollapsed = true;
+      hideSubmenuPanelImmediately();
     } else {
       state.isSidebarCollapsed = false;
+      showSubmenuPanelImmediately();
     }
   }
 
@@ -238,10 +280,16 @@ module.exports = function initUi(state, dom) {
       searchContainer.classList.remove('active');
       overlay.classList.remove('active');
       state.isSidebarOpen = false;
+      if (state.isSidebarCollapsed) {
+        hideSubmenuPanelImmediately();
+      } else {
+        showSubmenuPanelImmediately();
+      }
     } else {
       // 在移动设备下，重置侧边栏折叠状态
       sidebar.classList.remove('collapsed');
       content.classList.remove('expanded');
+      showSubmenuPanelImmediately();
     }
   });
 
diff --git a/test/sidebar-submenu-panel-and-theme-mode.node-test.js b/test/sidebar-submenu-panel-and-theme-mode.node-test.js
index da032b2..a8a5c1b 100644
--- a/test/sidebar-submenu-panel-and-theme-mode.node-test.js
+++ b/test/sidebar-submenu-panel-and-theme-mode.node-test.js
@@ -44,3 +44,26 @@ test('侧边栏样式：收起时不应在页面按钮下方显示目录子菜
     '收起态的 submenu 应明确设置为 display: none'
   );
 });
+
+test('侧边栏目录面板：应默认隐藏，并通过独立状态类延后淡入', () => {
+  const repoRoot = path.resolve(__dirname, '..');
+  const sidebarStylePath = path.join(repoRoot, 'assets', 'styles', '_sidebar.css');
+  const uiPath = path.join(repoRoot, 'src', 'runtime', 'app', 'ui.js');
+  const styleContent = fs.readFileSync(sidebarStylePath, 'utf8');
+  const uiContent = fs.readFileSync(uiPath, 'utf8');
+
+  assert.match(
+    styleContent,
+    /\.sidebar-submenu-panel\s*\{[\s\S]*opacity:\s*0;[\s\S]*visibility:\s*hidden;/m,
+    '目录面板默认应处于隐藏状态'
+  );
+  assert.match(
+    styleContent,
+    /\.sidebar\.submenu-panel-visible \.sidebar-submenu-panel:not\(:empty\)\s*\{[\s\S]*opacity:\s*1;[\s\S]*visibility:\s*visible;/m,
+    '目录面板应通过 submenu-panel-visible 状态类显现'
+  );
+  assert.ok(
+    uiContent.includes('submenu-panel-visible'),
+    'src/runtime/app/ui.js 应控制 submenu-panel-visible 状态类'
+  );
+});