Stable calculation of active toc (#1456)

Fix this unstable active table-of-contents link calculation shown in
this video:
Clicking "Plugin types", active toc link will be "Option Parsing" or
"File Parsing", depend on previous scroll position.

https://github.com/user-attachments/assets/bfd5c92a-9f11-4c9b-9267-86ae55607541


IntersectionObserver only notifies changed intersections.
We need to track which heading tag currently intersects with viewport.
(I use Set in this pull request)
Use the top-most intersecting heading to make user-clicked toc links
match with active toc links.

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Author: tompng

lib/rdoc/generator/template/aliki/js/aliki.js

@@ -209,54 +209,65 @@ function generateToc() {
 
 function hookTocActiveHighlighting() {
   var tocLinks = document.querySelectorAll('.toc-link');
-  if (tocLinks.length === 0) return;
+  var targetHeadings = [];
+  tocLinks.forEach(function(link) {
+    var targetId = link.getAttribute('data-target');
+    var heading = document.getElementById(targetId);
+    if (heading) {
+      targetHeadings.push(heading);
+    }
+  });
+
+  if (targetHeadings.length === 0) return;
 
   var observerOptions = {
     root: null,
-    rootMargin: '-20% 0px -35% 0px',
+    rootMargin: '0% 0px -35% 0px',
     threshold: 0
   };
 
-  var activeLink = null;
+  var intersectingHeadings = new Set();
+  function update() {
+    var firstIntersectingHeading = targetHeadings.find(function(heading) {
+      return intersectingHeadings.has(heading);
+    });
+    if (!firstIntersectingHeading) return;
+    var correspondingLink = document.querySelector('.toc-link[data-target="' + firstIntersectingHeading.id + '"]');
+    if (!correspondingLink) return;
+
+    // Remove active class from all links
+    tocLinks.forEach(function(link) {
+      link.classList.remove('active');
+    });
+
+    // Add active class to current link
+    correspondingLink.classList.add('active');
+
+    // Scroll link into view if needed
+    var tocNav = document.querySelector('#toc-nav');
+    if (tocNav) {
+      var linkRect = correspondingLink.getBoundingClientRect();
+      var navRect = tocNav.getBoundingClientRect();
 
+      if (linkRect.top < navRect.top || linkRect.bottom > navRect.bottom) {
+        correspondingLink.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
+      }
+    }
+  }
   var observer = new IntersectionObserver(function(entries) {
     entries.forEach(function(entry) {
       if (entry.isIntersecting) {
-        var id = entry.target.id;
-        var correspondingLink = document.querySelector('.toc-link[data-target="' + id + '"]');
-
-        if (correspondingLink) {
-          // Remove active class from all links
-          tocLinks.forEach(function(link) {
-            link.classList.remove('active');
-          });
-
-          // Add active class to current link
-          correspondingLink.classList.add('active');
-          activeLink = correspondingLink;
-
-          // Scroll link into view if needed
-          var tocNav = document.querySelector('#toc-nav');
-          if (tocNav) {
-            var linkRect = correspondingLink.getBoundingClientRect();
-            var navRect = tocNav.getBoundingClientRect();
-
-            if (linkRect.top < navRect.top || linkRect.bottom > navRect.bottom) {
-              correspondingLink.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
-            }
-          }
-        }
+        intersectingHeadings.add(entry.target);
+      } else {
+        intersectingHeadings.delete(entry.target);
       }
     });
+    update();
   }, observerOptions);
 
   // Observe all headings that have corresponding TOC links
-  tocLinks.forEach(function(link) {
-    var targetId = link.getAttribute('data-target');
-    var targetHeading = document.getElementById(targetId);
-    if (targetHeading) {
-      observer.observe(targetHeading);
-    }
+  targetHeadings.forEach(function(heading) {
+    observer.observe(heading);
   });
 
   // Smooth scroll when clicking TOC links