perf: stop heading lookup at the requested occurrence (#20246)

Co-authored-by: Orca Worker <orca-worker@localhost>

src/renderer/src/components/editor/rich-markdown-toc-heading-target.test.ts

@@ -14,6 +14,14 @@ function makeTocItem(
 }
 
 describe('findRichMarkdownTocHeadingTarget', () => {
+  it('returns undefined when the requested duplicate occurrence is missing', () => {
+    const container = document.createElement('div')
+    container.innerHTML = '<h2>Repeat</h2><h2>Other</h2>'
+    const items = [makeTocItem('repeat', 2, 'Repeat'), makeTocItem('repeat-1', 2, 'Repeat')]
+    expect(findRichMarkdownTocHeadingTarget(container, items, 'repeat-1')).toBeUndefined()
+    expect(findRichMarkdownTocHeadingTarget(container, items, 'missing')).toBeUndefined()
+  })
+
   it('finds deep headings that the table of contents exposes', () => {
     const container = document.createElement('div')
     container.innerHTML = `

src/renderer/src/components/editor/rich-markdown-toc-heading-target.ts

@@ -18,9 +18,16 @@ export function findRichMarkdownTocHeadingTarget(
   const sameTitleIndex = items
     .filter((item) => item.title === target.title)
     .findIndex((item) => item.id === target.id)
-  const matchingHeadings = Array.from(
-    container.querySelectorAll<HTMLElement>(RICH_MARKDOWN_TOC_HEADING_SELECTOR)
-  ).filter((candidate) => candidate.textContent?.trim() === target.title)
-
-  return matchingHeadings.at(Math.max(0, sameTitleIndex))
+  let remaining = Math.max(0, sameTitleIndex)
+  for (const candidate of container.querySelectorAll<HTMLElement>(
+    RICH_MARKDOWN_TOC_HEADING_SELECTOR
+  )) {
+    if (candidate.textContent?.trim() === target.title) {
+      if (remaining === 0) {
+        return candidate
+      }
+      remaining -= 1
+    }
+  }
+  return undefined
 }