Mark sidebar toc entry corresponding to current hash

Also scroll it to the top on initial page load

docs/_static/custom.css

@@ -13,3 +13,7 @@
     margin-bottom: 0.75ex;
     margin-top: 0.75ex;
 }
+
+.sidebar-tree a.current {
+    font-style: italic;
+}

docs/_static/custom.js

@@ -0,0 +1,45 @@
+/* vim:fileencoding=utf-8
+ * 
+ * Copyright (C) 2021 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPLv3 license
+ */
+
+(function() {
+"use strict";
+
+function get_sidebar_tree() {
+    return document.querySelector('.sidebar-tree');
+}
+
+function mark_current_link(sidebar_tree, a, onload) {
+    console.log(onload);
+    var li = a.closest('li.has-children');
+    while (li) {
+        li.querySelector('input[type=checkbox]').setAttribute('checked', 'checked');
+        li = li.parentNode.closest('li.has-children');
+    }
+    sidebar_tree.querySelectorAll('.current').forEach(function (elem) {
+        elem.classList.remove('current');
+    });
+    if (onload) a.scrollIntoView();
+    a.classList.add('current');
+}
+
+function show_hash_in_sidebar(onload) {
+    var sidebar_tree = document.querySelector('.sidebar-tree');
+    if (document.location.hash.length > 1) {
+        var a = sidebar_tree.querySelector('a[href="' + document.location.hash + '"]');
+        if (a) mark_current_link(sidebar_tree, a, onload);
+    } else {
+        if (onload) sidebar_tree.querySelector('.current-page a').scrollIntoView();
+    }
+}
+
+document.addEventListener("DOMContentLoaded", function() {
+    show_hash_in_sidebar(true);
+    window.addEventListener('hashchange', show_hash_in_sidebar.bind(null, false));
+});
+
+}());
+

docs/conf.py

@@ -131,6 +131,7 @@ html_theme_options: Dict[str, Any] = {
 html_static_path = ['_static']
 html_favicon = html_logo = '../logo/kitty.svg'
 html_css_files = ['custom.css']
+html_js_files = ['custom.js']
 
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.