diff --git a/source/css/_common/outline/sidebar/sidebar-nav.styl b/source/css/_common/outline/sidebar/sidebar-nav.styl
index 5a49bee61..eabfb8942 100644
--- a/source/css/_common/outline/sidebar/sidebar-nav.styl
+++ b/source/css/_common/outline/sidebar/sidebar-nav.styl
@@ -1,12 +1,19 @@
 // Sidebar Navigation
 .sidebar-nav {
-  display: none;
+  font-size: $font-size-small;
+  height: 0;
   margin: 0;
-  padding-bottom: 20px;
+  overflow: hidden;
   padding-left: 0;
+  pointer-events: none;
+  transition: $transition-ease;
+  transition-property: height, visibility;
+  visibility: hidden;
 
   .sidebar-nav-active & {
-    display: block;
+    height: "calc(%sem + 1px)" % $line-height-base;
+    pointer-events: unset;
+    visibility: unset;
   }
 
   li {
@@ -14,7 +21,8 @@
     color: $sidebar-nav-color;
     cursor: pointer;
     display: inline-block;
-    font-size: $font-size-small;
+    transition: $transition-ease;
+    transition-property: border-bottom-color, color;
 
     &.sidebar-nav-overview {
       margin-left: 10px;
@@ -29,30 +37,93 @@
 .sidebar-toc-active .sidebar-nav-toc, .sidebar-overview-active .sidebar-nav-overview {
   border-bottom-color: $sidebar-highlight;
   color: $sidebar-highlight;
+  transition-delay: $transition-duration;
 
   &:hover {
     color: $sidebar-highlight;
   }
 }
 
-// Need for Sidebar/TOC inner scrolling if content taller then viewport.
+// For TOC/Overview scrolling
 .sidebar-panel-container {
+  align-items: start;
+  display: grid;
   flex: 1;
   overflow-x: hidden;
   overflow-y: auto;
+  padding-top: 0;
+  transition: padding-top $transition-ease;
+
+  .sidebar-nav-active & {
+    padding-top: 20px;
+  }
 }
 
 .sidebar-panel {
-  display: none;
+  animation: deactivate-sidebar-panel $transition-duration ease-in-out;
+  grid-area: 1 / 1;
+  height: 0;
+  opacity: 0;
+  overflow: hidden;
+  pointer-events: none;
+  transform: translateY(0);
+  transition: $transition-ease;
+  transition-delay: 0s;
+  transition-property: opacity, transform, visibility;
+  visibility: hidden;
+
+  // Apply transform to both panels when sidebar nav is active,
+  // to the TOC panel when switching between Overview and TOC regardless of
+  // whether the sidebar nav is active
+  .sidebar-nav-active &,
+  .sidebar-overview-active &.post-toc-wrap {
+    transform: translateY(-20px);
+  }
+
+  // Delay TOC transform transition when switching from TOC to Overview and
+  // deactivating the sidebar nav at the same time, to prevent the TOC panel
+  // from moving too fast
+  // https://github.com/next-theme/hexo-theme-next/pull/323#issuecomment-1420780965
+  .sidebar-overview-active:not(.sidebar-nav-active) &.post-toc-wrap {
+    transition-delay: 0s, $transition-duration, 0s;
+  }
+
+  .sidebar-overview-active &.site-overview-wrap,
+  .sidebar-toc-active &.post-toc-wrap {
+    animation-name: activate-sidebar-panel;
+    height: auto;
+    opacity: 1;
+    pointer-events: unset;
+    transform: translateY(0);
+    // The visibility delay is intentionally set to 0s to accommodate
+    // the visibility change on initial page load.
+    transition-delay: $transition-duration, $transition-duration, 0s;
+    visibility: unset;
+  }
+
+  &.site-overview-wrap {
+    // Flexbox layout makes it possible to reorder the child
+    // elements of .site-overview-wrap through the `order` CSS property
+    flex-column();
+    gap: 10px;
+    justify-content: flex-start; // TODO: Optimize the duplicate with flex-column()
+  }
 }
 
-.sidebar-overview-active .site-overview-wrap {
-  // Flexbox layout makes it possible to reorder the child
-  // elements of .site-overview-wrap through the `order` CSS property
-  flex-column();
-  gap: 10px;
+@keyframes deactivate-sidebar-panel {
+  from {
+    height: var(--inactive-panel-height, 0);
+  }
+  to {
+    height: var(--active-panel-height, 0);
+  }
 }
 
-.sidebar-toc-active .post-toc-wrap {
-  display: block;
+@keyframes activate-sidebar-panel {
+  from {
+    height: var(--inactive-panel-height, auto);
+  }
+  to {
+    height: var(--active-panel-height, auto);
+  }
 }
diff --git a/source/css/_common/outline/sidebar/sidebar-toc.styl b/source/css/_common/outline/sidebar/sidebar-toc.styl
index 18f47292e..253191dc3 100644
--- a/source/css/_common/outline/sidebar/sidebar-toc.styl
+++ b/source/css/_common/outline/sidebar/sidebar-toc.styl
@@ -5,9 +5,13 @@ if (hexo-config('toc.enable')) {
     ol {
       list-style: none;
       margin: 0;
-      padding: 0 2px 5px 10px;
+      padding: 0 2px 0 10px;
       text-align: left;
 
+      > :last-child {
+        margin-bottom: 5px;
+      }
+
       > ol {
         padding-left: 0;
       }
@@ -28,19 +32,21 @@ if (hexo-config('toc.enable')) {
     }
 
     .nav {
-      .nav-child {
-        display: hexo-config('toc.expand_all') ? block : none;
-      }
-
-      .active > .nav-child {
-        display: block;
-      }
-
-      .active-current > .nav-child {
-        display: block;
+      if (not hexo-config('toc.expand_all')) {
+        .nav-child {
+          --height: 0;
+          height: 0;
+          opacity: 0;
+          overflow: hidden;
+          transition-property: height, opacity, visibility;
+          transition: $transition-ease;
+          visibility: hidden;
+        }
 
-        > .nav-item {
-          display: block;
+        .active > .nav-child {
+          height: var(--height, auto);
+          opacity: 1;
+          visibility: unset;
         }
       }
 
diff --git a/source/css/_variables/base.styl b/source/css/_variables/base.styl
index 96515f8cf..e7fb8c359 100644
--- a/source/css/_variables/base.styl
+++ b/source/css/_variables/base.styl
@@ -24,9 +24,10 @@ $orange       = #fc6423;
 
 // Transition
 // --------------------------------------------------
-$transition-ease     = .2s ease-in-out;
-$transition-ease-in  = .2s ease-in;
-$transition-ease-out = .2s ease-out;
+$transition-duration = .2s;
+$transition-ease     = $transition-duration ease-in-out;
+$transition-ease-in  = $transition-duration ease-in;
+$transition-ease-out = $transition-duration ease-out;
 
 
 // Scaffolding
diff --git a/source/js/pjax.js b/source/js/pjax.js
index ec24341d7..f81a6a0b1 100644
--- a/source/js/pjax.js
+++ b/source/js/pjax.js
@@ -4,11 +4,25 @@ const pjax = new Pjax({
   selectors: [
     'head title',
     'script[type="application/json"]',
-    '.main-inner',
+    // Precede .main-inner to prevent placeholder TOC changes asap
     '.post-toc-wrap',
+    '.main-inner',
     '.languages',
     '.pjax'
   ],
+  switches: {
+    '.post-toc-wrap': function(oldWrap, newWrap) {
+      if (newWrap.querySelector('.post-toc')) {
+        Pjax.switches.outerHTML.call(this, oldWrap, newWrap);
+      } else {
+        const curTOC = oldWrap.querySelector('.post-toc');
+        if (curTOC) {
+          curTOC.classList.add('placeholder-toc');
+        }
+        this.onSwitch();
+      }
+    }
+  },
   analytics: false,
   cacheBust: false,
   scrollTo : !CONFIG.bookmark.enable
@@ -28,7 +42,7 @@ document.addEventListener('pjax:success', () => {
       .bootstrap();
   }
   if (CONFIG.sidebar.display !== 'remove') {
-    const hasTOC = document.querySelector('.post-toc');
+    const hasTOC = document.querySelector('.post-toc:not(.placeholder-toc)');
     document.querySelector('.sidebar-inner').classList.toggle('sidebar-nav-active', hasTOC);
     NexT.utils.activateSidebarPanel(hasTOC ? 0 : 1);
     NexT.utils.updateSidebarPosition();
diff --git a/source/js/utils.js b/source/js/utils.js
index 949cd97f3..ee10f6198 100644
--- a/source/js/utils.js
+++ b/source/js/utils.js
@@ -122,6 +122,19 @@ NexT.utils = {
     });
   },
 
+  updateActiveNav: function() {
+    if (!Array.isArray(NexT.utils.sections)) return;
+    let index = NexT.utils.sections.findIndex(element => {
+      return element && element.getBoundingClientRect().top > 10;
+    });
+    if (index === -1) {
+      index = NexT.utils.sections.length - 1;
+    } else if (index > 0) {
+      index--;
+    }
+    this.activateNavByIndex(index);
+  },
+
   registerScrollPercent: function() {
     const backToTop = document.querySelector('.back-to-top');
     const readingProgressBar = document.querySelector('.reading-progress-bar');
@@ -138,16 +151,7 @@ NexT.utils = {
           readingProgressBar.style.setProperty('--progress', scrollPercent.toFixed(2) + '%');
         }
       }
-      if (!Array.isArray(NexT.utils.sections)) return;
-      let index = NexT.utils.sections.findIndex(element => {
-        return element && element.getBoundingClientRect().top > 10;
-      });
-      if (index === -1) {
-        index = NexT.utils.sections.length - 1;
-      } else if (index > 0) {
-        index--;
-      }
-      this.activateNavByIndex(index);
+      this.updateActiveNav();
     }, { passive: true });
 
     backToTop && backToTop.addEventListener('click', () => {
@@ -263,7 +267,7 @@ NexT.utils = {
   },
 
   registerSidebarTOC: function() {
-    this.sections = [...document.querySelectorAll('.post-toc li a.nav-link')].map(element => {
+    this.sections = [...document.querySelectorAll('.post-toc:not(.placeholder-toc) li a.nav-link')].map(element => {
       const target = document.getElementById(decodeURI(element.getAttribute('href')).replace('#', ''));
       // TOC item animation navigate.
       element.addEventListener('click', event => {
@@ -281,6 +285,7 @@ NexT.utils = {
       });
       return target;
     });
+    this.updateActiveNav();
   },
 
   registerPostReward: function() {
@@ -292,18 +297,35 @@ NexT.utils = {
   },
 
   activateNavByIndex: function(index) {
-    const target = document.querySelectorAll('.post-toc li a.nav-link')[index];
+    const nav = document.querySelector('.post-toc:not(.placeholder-toc) .nav');
+    if (!nav) return;
+
+    const navItemList = nav.querySelectorAll('.nav-item');
+    const target = navItemList[index];
     if (!target || target.classList.contains('active-current')) return;
 
-    document.querySelectorAll('.post-toc .active').forEach(element => {
-      element.classList.remove('active', 'active-current');
+    const singleHeight = navItemList[navItemList.length - 1].offsetHeight;
+
+    nav.querySelectorAll('.active').forEach(navItem => {
+      navItem.classList.remove('active', 'active-current');
     });
     target.classList.add('active', 'active-current');
-    let parent = target.parentNode;
-    while (!parent.matches('.post-toc')) {
-      if (parent.matches('li')) parent.classList.add('active');
-      parent = parent.parentNode;
+
+    let activateEle = target.querySelector('.nav-child') || target.parentElement;
+    let navChildHeight = 0;
+
+    while (nav.contains(activateEle)) {
+      if (activateEle.classList.contains('nav-item')) {
+        activateEle.classList.add('active');
+      } else { // .nav-child or .nav
+        // scrollHeight isn't reliable for transitioning child items.
+        // The last nav-item in a list has a margin-bottom of 5px.
+        navChildHeight += (singleHeight * activateEle.childElementCount) + 5;
+        activateEle.style.setProperty('--height', `${navChildHeight}px`);
+      }
+      activateEle = activateEle.parentElement;
     }
+
     // Scrolling to center active TOC element if TOC content is taller then viewport.
     const tocElement = document.querySelector(CONFIG.scheme === 'Pisces' || CONFIG.scheme === 'Gemini' ? '.sidebar-panel-container' : '.sidebar');
     if (!document.querySelector('.sidebar-toc-active')) return;
@@ -318,7 +340,7 @@ NexT.utils = {
   updateSidebarPosition: function() {
     if (window.innerWidth < 1200 || CONFIG.scheme === 'Pisces' || CONFIG.scheme === 'Gemini') return;
     // Expand sidebar on post detail page by default, when post has a toc.
-    const hasTOC = document.querySelector('.post-toc');
+    const hasTOC = document.querySelector('.post-toc:not(.placeholder-toc)');
     let display = CONFIG.page.sidebar;
     if (typeof display !== 'boolean') {
       // There's no definition sidebar in the page front-matter.
@@ -330,31 +352,30 @@ NexT.utils = {
   },
 
   activateSidebarPanel: function(index) {
-    const duration = 200;
     const sidebar = document.querySelector('.sidebar-inner');
-    const panel = document.querySelector('.sidebar-panel-container');
-    const activeClassName = ['sidebar-toc-active', 'sidebar-overview-active'];
+    const activeClassNames = ['sidebar-toc-active', 'sidebar-overview-active'];
+    if (sidebar.classList.contains(activeClassNames[index])) return;
 
-    if (sidebar.classList.contains(activeClassName[index])) return;
+    const panelContainer = sidebar.querySelector('.sidebar-panel-container');
+    const tocPanel = panelContainer.firstElementChild;
+    const overviewPanel = panelContainer.lastElementChild;
 
-    window.anime({
-      duration,
-      targets   : panel,
-      easing    : 'linear',
-      opacity   : 0,
-      translateY: [0, -20],
-      complete  : () => {
-        // Prevent adding TOC to Overview if Overview was selected when close & open sidebar.
-        sidebar.classList.replace(activeClassName[1 - index], activeClassName[index]);
-        window.anime({
-          duration,
-          targets   : panel,
-          easing    : 'linear',
-          opacity   : [0, 1],
-          translateY: [-20, 0]
-        });
+    let postTOCHeight = tocPanel.scrollHeight;
+    // For TOC activation, try to use the animated TOC height
+    if (index === 0) {
+      const nav = tocPanel.querySelector('.nav');
+      if (nav) {
+        postTOCHeight = parseInt(nav.style.getPropertyValue('--height'), 10);
       }
-    });
+    }
+    const panelHeights = [
+      postTOCHeight,
+      overviewPanel.scrollHeight
+    ];
+    panelContainer.style.setProperty('--inactive-panel-height', `${panelHeights[1 - index]}px`);
+    panelContainer.style.setProperty('--active-panel-height', `${panelHeights[index]}px`);
+
+    sidebar.classList.replace(activeClassNames[1 - index], activeClassNames[index]);
   },
 
   getScript: function(src, options = {}, legacyCondition) {