Fix duplicated TOC entries when chapter starts with h2+

When a chapter's markdown file starts with an h2 or lower heading (e.g.
`## Section Title`), the 'on this page' sidebar feature would add it as
a sub-entry, duplicating the chapter's existing SUMMARY.md sidebar link.

This skips the first heading in the on-this-page list when its text
matches the active sidebar link, preventing the duplication.

Fixes #2995

Author: cobyfrombrooklyn-bot

crates/mdbook-html/front-end/templates/toc.js.hbs

@@ -356,6 +356,16 @@ window.customElements.define('mdbook-sidebar-scrollbox', MDBookSidebarScrollbox)
         headers = Array.from(main.querySelectorAll('h2, h3, h4, h5, h6'))
             .filter(h => h.id !== '' && h.children.length && h.children[0].tagName === 'A');
 
+        // Skip the first heading if its text matches the active sidebar link.
+        // This avoids duplicating the chapter title in the "on this page" list
+        // when a chapter's markdown starts with a heading (e.g. `## Section Title`)
+        // that matches its SUMMARY.md entry.
+        // See https://github.com/rust-lang/mdBook/issues/2995
+        if (headers.length > 0
+            && activeSection.textContent.trim() === headers[0].textContent.trim()) {
+            headers.shift();
+        }
+
         if (headers.length === 0) {
             return;
         }

tests/testsuite/toc.rs

@@ -141,6 +141,17 @@ fn check_link_target_fallback() {
     );
 }
 
+/// The "on this page" feature should skip the first heading if it matches
+/// the active sidebar link text, to avoid duplicating the chapter title
+/// when a chapter starts with an h2+ heading.
+/// See <https://github.com/rust-lang/mdBook/issues/2995>.
+#[test]
+fn toc_js_deduplicates_first_heading() {
+    BookTest::from_dir("toc/basic_toc")
+        .check_file_contains("book/toc*.js", "activeSection.textContent.trim() === headers[0].textContent.trim()")
+        .check_file_contains("book/toc*.js", "headers.shift()");
+}
+
 // Checks formatting of summary names with inline elements.
 #[test]
 fn summary_with_markdown_formatting() {