made left sidebar sticky and higlight current chapter

css/style.css

@@ -1033,8 +1033,9 @@ footer.footer-modern a:hover {
 .toc-sidebar {
 	margin-top: var(--space-xl);
 	position: sticky;
-	top: var(--space-lg);
+	/* 56px = fixed navbar height; extra gap so the ToC doesn't hug it */
-	max-height: calc(100vh - var(--space-xl));
+	top: calc(56px + var(--space-sm));
+	max-height: calc(100vh - 56px - var(--space-md));
 	overflow-y: auto;
 	font-size: var(--font-size-sm);
 }

index.php

@@ -106,8 +106,9 @@
 
 			<?php if ($hasHeadings): ?>
 
-			<!-- Left ToC Sidebar — visible on md+ only; hidden on mobile -->
+			<!-- Left ToC Sidebar — visible on md+ only; hidden on mobile.
-			<div class="col-md-3 d-none d-md-block align-self-start">
+			     No align-self-start so the column stretches to article height, enabling sticky. -->
+			<div class="col-md-3 d-none d-md-block">
 			<?php include(THEME_DIR_PHP.'toc.php'); ?>
 			</div>