diff --git a/extension/content.js b/extension/content.js
index 7b6bd93..c9d6eca 100644
--- a/extension/content.js
+++ b/extension/content.js
@@ -3521,19 +3521,7 @@ function findActiveSubtitleIndex(currentTime) {
 }
 
 function findActiveChapterIndex(currentTime) {
-  const chapters = MdgSites.normalizeChapters(state.chapters || []);
-  for (let index = 0; index < chapters.length; index += 1) {
-    const item = chapters[index];
-    const from = Number(item?.from || 0) || 0;
-    const next = chapters[index + 1];
-    const explicitTo = Number(item?.to || 0) || 0;
-    const fallbackTo = next && Number(next.from) > from ? Number(next.from) : explicitTo;
-    const to = fallbackTo > from ? fallbackTo : Number.POSITIVE_INFINITY;
-    if (currentTime >= from && currentTime < to) {
-      return index;
-    }
-  }
-  return -1;
+  return MdgSites.chapterRanges(state.chapters || []).findIndex((item) => currentTime >= item.from && currentTime < item.to);
 }
 
 function setActiveReadingItems(subtitleIndex, chapterIndex, shouldScroll = false) {
diff --git a/extension/content.selftest.js b/extension/content.selftest.js
index 6b3b32e..76e53a7 100644
--- a/extension/content.selftest.js
+++ b/extension/content.selftest.js
@@ -492,6 +492,14 @@ const settle = async () => {
     t.byId = prev;
   }
 
+  // 专注模式 highlights the chapter the note files the line under: two chapters that start together end at the next later start.
+  {
+    const saved = state.chapters;
+    state.chapters = [{ title: "A", from: 0, to: 0 }, { title: "B", from: 0, to: 0 }, { title: "C", from: 60, to: 0 }];
+    assert.deepStrictEqual([10, 90].map(t.findActiveChapterIndex), [0, 2]);
+    state.chapters = saved;
+  }
+
   // A4: a jump from a 专注模式 subtitle row or chapter keeps a paused video paused, like the side panel's timestamps.
   {
     const prev = t.bindReadingViewVideo;
diff --git a/extension/note.js b/extension/note.js
index c4b977e..cf18c9d 100644
--- a/extension/note.js
+++ b/extension/note.js
@@ -635,7 +635,7 @@
       return [];
     }
 
-    const chapterItems = MdgSites.normalizeChapters(chapters);
+    const chapterItems = MdgSites.chapterRanges(chapters);
     if (chapterItems.length === 0) {
       return subtitleItems.map((item) => formatSubtitleLine(item, settings, withHours));
     }
@@ -653,26 +653,16 @@
       lines.push("");
     }
 
-    chapterItems.forEach((chapter, idx) => {
-      const start = Number(chapter.from || 0) || 0;
-      // Chapters that start together: the first one ends where a later start is; each subtitle goes in one chapter.
-      const next = chapterItems.slice(idx + 1).find((item) => Number(item.from) > start);
-      const chapterTo = Number(chapter.to || 0) || 0;
-      let end = Infinity;
-      if (next) {
-        end = Number(next.from);
-      } else if (chapterTo > start) {
-        end = chapterTo;
-      }
+    chapterItems.forEach((chapter) => {
+      // Chapters that start together share a range; usedIndexes puts each subtitle in the first of them.
+      const { from: start, to: end } = chapter;
 
       const sectionItems = subtitleItems.filter((item) => {
         if (usedIndexes.has(item._index)) {
           return false;
         }
         const from = Number(item.from || 0) || 0;
-        const inStart = from + 0.001 >= start;
-        const inEnd = end === Infinity ? true : from < end;
-        return inStart && inEnd;
+        return from + 0.001 >= start && from < end;
       });
 
       if (sectionItems.length === 0) {
diff --git a/extension/sites.js b/extension/sites.js
index 4f3a235..7fc195f 100644
--- a/extension/sites.js
+++ b/extension/sites.js
@@ -86,6 +86,15 @@
     });
   }
 
+  // Normalized chapters with `to` as where each one ends: the first later start, else its own `to`, else Infinity.
+  // Chapters that start together all end at the next later start, never at each other.
+  function chapterRanges(chapters) {
+    return normalizeChapters(chapters).map((item, index, all) => {
+      const next = all.slice(index + 1).find((later) => later.from > item.from);
+      return { ...item, to: next ? next.from : item.to > item.from ? item.to : Infinity };
+    });
+  }
+
   function trackLanguagePriority(track) {
     const lang = String(track?.lang || "").toLowerCase();
     const label = String(track?.label || "").toLowerCase();
@@ -1611,6 +1620,7 @@
     biliWbiSigned,
     biliWbiReset,
     normalizeChapters,
+    chapterRanges,
     parseChaptersFromDescription,
     decodeXmlEntities,
     parseSrv3,