From a73e07eac06a70f63896e2bb7f0c205534dc21c8 Mon Sep 17 00:00:00 2001
From: Luffy <lufei@apache.org>
Date: Mon, 6 Jan 2025 11:43:40 +0800
Subject: [PATCH] fix: Prevent initial unnecessary IntersectionObserver
 callback execution (#2523)

---
 src/core/event/index.js     | 39 ++++++++++++++++++++++++-------------
 src/core/render/compiler.js |  2 +-
 2 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/src/core/event/index.js b/src/core/event/index.js
index ef8a39b4c..fe4076f05 100644
--- a/src/core/event/index.js
+++ b/src/core/event/index.js
@@ -45,7 +45,7 @@ export function Events(Base) {
     // =========================================================================
     /**
      * Initialize cover observer
-     * Toggles sticky behavior when when cover is not in view
+     * Toggles sticky behavior when cover is not in view
      * @void
      */
     #initCover() {
@@ -74,11 +74,17 @@ export function Events(Base) {
     #initHeadings() {
       const headingElms = dom.findAll('#main :where(h1, h2, h3, h4, h5)');
       const headingsInView = new Set();
+      let isInitialLoad = true;
 
       // Mark sidebar active item on heading intersection
       this.#intersectionObserver?.disconnect();
       this.#intersectionObserver = new IntersectionObserver(
         entries => {
+          if (isInitialLoad) {
+            isInitialLoad = false;
+            return;
+          }
+
           if (this.#isScrolling) {
             return;
           }
@@ -89,18 +95,25 @@ export function Events(Base) {
             headingsInView[op](entry.target);
           }
 
-          const activeHeading =
-            headingsInView.size > 1
-              ? // Sort headings by proximity to viewport top and select first
-                Array.from(headingsInView).sort((a, b) =>
-                  a.compareDocumentPosition(b) &
-                  Node.DOCUMENT_POSITION_FOLLOWING
-                    ? -1
-                    : 1,
-                )[0]
-              : // Get first and only item in set.
-                // May be undefined if no headings are in view.
-                headingsInView.values().next().value;
+          let activeHeading;
+          if (headingsInView.size === 1) {
+            // Get first and only item in set.
+            // May be undefined if no headings are in view.
+            activeHeading = headingsInView.values().next().value;
+          } else if (headingsInView.size > 1) {
+            // Find the closest heading to the top of the viewport
+            // Reduce over the Set of headings currently in view (headingsInView) to determine the closest heading.
+            activeHeading = Array.from(headingsInView).reduce(
+              (closest, current) => {
+                return !closest ||
+                  closest.compareDocumentPosition(current) &
+                    Node.DOCUMENT_POSITION_FOLLOWING
+                  ? current
+                  : closest;
+              },
+              null,
+            );
+          }
 
           if (activeHeading) {
             const id = activeHeading.getAttribute('id');
diff --git a/src/core/render/compiler.js b/src/core/render/compiler.js
index b86166301..124be9b7f 100644
--- a/src/core/render/compiler.js
+++ b/src/core/render/compiler.js
@@ -181,7 +181,7 @@ export class Compiler {
   }
 
   /**
-   * Compile sidebar, it uses  _sidebar.md ( or specific file) or the content's headings toc to render sidebar.
+   * Compile sidebar, it uses _sidebar.md (or specific file) or the content's headings toc to render sidebar.
    * @param {String} text Text content from the sidebar file, maybe empty
    * @param {Number} level Type of heading (h<level> tag)
    * @returns {String} Sidebar element