change the position absolute to the position fixed on the responsive md, sm and xs

src/css/Sidebar.styl

@@ -156,7 +156,7 @@ leftside()
 @media (min-width: min-sm) and (max-width: max-md)
 
   .sidebar
-    position absolute
+    position fixed
 
 /***** MEDIA 576px and 767px *****/
 
@@ -173,4 +173,4 @@ leftside()
     padding-top 69px
 
   .sidebar
-    position absolute
+    position fixed