feat: use position: sticky for discussion side nav (#3540)

framework/core/less/common/App.less

@@ -2,7 +2,6 @@
   position: relative !important;
   padding-top: var(--header-height);
   padding-bottom: 50px;
-  overflow-x: hidden;
   min-height: 100vh;
 
   @media @phone {

framework/core/less/common/scaffolding.less

@@ -13,6 +13,7 @@ body {
   font-size: 13px;
   line-height: 1.5;
   overflow-y: scroll;
+  overflow-x: hidden;
 }
 
 h1, h2, h3, h4, h5, h6 {

framework/core/less/forum/DiscussionPage.less

@@ -25,15 +25,15 @@
 @media @tablet-up {
   .DiscussionPage-nav {
     float: right;
+    position: sticky;
+    top: var(--header-height);
+    padding-top: 32px;
 
     &, > ul {
       width: 150px;
     }
+    
     > ul {
-      position: fixed;
-      margin-top: 30px;
-      z-index: 1;
-
       > li {
         margin-bottom: 10px;
       }

framework/core/less/forum/Post.less

@@ -8,7 +8,6 @@
   position: relative;
   top: 0;
   border-radius: var(--border-radius);
-  .clearfix();
 
   &.editing {
     top: 5px;