Fix flicker when setting sidebar scroll position

Previously, sidebar scroll was set in an external script which caused a
flicker as the sidebar is initially rendered without any scroll before
being scrolled to the desired location.

Switching to an inline script right after the HTML tags for the sidebar
seems to avoid the flicker in most cases. In addition, logic is added to
avoid scrolling jumps when navigating via links within the sidebar.

src/theme/book.js

@@ -551,13 +551,6 @@ function playground_text(playground, hidden = true) {
             firstContact = null;
         }
     }, { passive: true });
-
-    // Scroll sidebar to current active section
-    var activeSection = document.getElementById("sidebar").querySelector(".active");
-    if (activeSection) {
-        // https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollIntoView
-        activeSection.scrollIntoView({ block: 'center' });
-    }
 })();
 
 (function chapterNavigation() {

src/theme/index.hbs

@@ -110,6 +110,28 @@
             <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
         </nav>
 
+        <!-- Track and set sidebar scroll position -->
+        <script>
+            var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox');
+            sidebarScrollbox.addEventListener('click', function(e) {
+                if (e.target.tagName === 'A') {
+                    sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
+                }
+            }, { passive: true });
+            var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
+            sessionStorage.removeItem('sidebar-scroll');
+            if (sidebarScrollTop) {
+                // preserve sidebar scroll position when navigating via links within sidebar
+                sidebarScrollbox.scrollTop = sidebarScrollTop;
+            } else {
+                // scroll sidebar to current active section when navigating via "next/previous chapter" buttons
+                var activeSection = document.querySelector('#sidebar .active');
+                if (activeSection) {
+                    activeSection.scrollIntoView({ block: 'center' });
+                }
+            }
+        </script>
+
         <div id="page-wrapper" class="page-wrapper">
 
             <div class="page">