make sidebar sticky

Author: mathroc

source/_layouts/psr.twig

@@ -11,7 +11,7 @@
         </div>
     </div>
     {% if page.related %}
-    <div class="columns__column columns__column--4 columns__column--padding_left">
+    <div class="columns__column columns__column--4 columns__column--padding_left sidebar__wrapper">
         <nav class="sidebar">
             <h2 class="sidebar__title">Additional Info:</h2>
             <ul class="sidebar__list">

source/_sass/blocks/sidebar.scss

@@ -127,3 +127,8 @@
         }
     }
 }
+
+.sidebar__wrapper {
+    position: sticky;
+    top: 0;
+}