Fix error in ToC script at the bottom of the page (#2002)

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

assets/js/toc.js

@@ -107,6 +107,11 @@ function getCurrentHeading(headings, headerOffset) {
     index += 1;
   }
 
+  // At the bottom of the page we might not have a heading.
+  if (!currentHeading) {
+    currentHeading = prevHeading;
+  }
+
   return currentHeading;
 }
 

changelogs/internal/newsfragments/2002.clarification

@@ -0,0 +1 @@
+Improve the JS script to highlight the current ToC entry.