diff --git a/backend/src/routes/exam-prep.js b/backend/src/routes/exam-prep.js
index 578c0ab..112c8c8 100644
--- a/backend/src/routes/exam-prep.js
+++ b/backend/src/routes/exam-prep.js
@@ -630,13 +630,15 @@ function pickRandomByDifficulty(examKey, count, excludeSlugs) {
     ? `AND (subtopic IS NULL OR subtopic NOT IN (${exParams.map(() => '?').join(',')}))`
     : '';
 
+  const COLS = `id, task_idx, variant, task_type, text_html, figure_html, opts_json,
+             answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph`;
   const out = [];
+  const seen = new Set();
   for (let d = 1; d <= 5; d++) {
     const limit = dist[d - 1];
     if (limit === 0) continue;
     const sql = `
-      SELECT id, task_idx, variant, task_type, text_html, figure_html, opts_json,
-             answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph
+      SELECT ${COLS}
         FROM exam_tasks
        WHERE exam_key = ? AND task_type IN ('mc','open')
          AND difficulty = ?
@@ -646,8 +648,25 @@ function pickRandomByDifficulty(examKey, count, excludeSlugs) {
     const args = exParams
       ? [examKey, d, ...exParams, limit]
       : [examKey, d, limit];
+    for (const r of db.prepare(sql).all(...args)) { if (!seen.has(r.id)) { seen.add(r.id); out.push(r); } }
+  }
+  // Backfill to `count` from any difficulty — covers tracks whose tasks don't
+  // span all 5 difficulty levels (otherwise empty levels would shrink the batch).
+  if (out.length < count) {
+    const ids = [...seen];
+    const notIn = ids.length ? `AND id NOT IN (${ids.map(() => '?').join(',')})` : '';
+    const sql = `
+      SELECT ${COLS}
+        FROM exam_tasks
+       WHERE exam_key = ? AND task_type IN ('mc','open')
+         ${exClause}
+         ${notIn}
+       ORDER BY RANDOM()
+       LIMIT ?`;
+    const args = [examKey, ...(exParams || []), ...ids, count - out.length];
     out.push(...db.prepare(sql).all(...args));
   }
+  out.sort((a, b) => (a.difficulty || 0) - (b.difficulty || 0));
   return out;
 }
 
diff --git a/frontend/lesson.html b/frontend/lesson.html
index 4aae0ff..3a4f13d 100644
--- a/frontend/lesson.html
+++ b/frontend/lesson.html
@@ -276,7 +276,7 @@
     .lesson-nav-btn-prev { justify-content: flex-start; }
     .lesson-nav-btn-next { justify-content: flex-end; margin-left: auto; }
     .lesson-nav-btn-label { font-size: 0.7rem; font-weight: 600; color: var(--text-3); display: block; }
-    .lesson-nav-btn-title { white-space: nowrap; overflow: hidden; text-overflow: ellipsis; max-width: 160px; }
+    .lesson-nav-btn-title { display: block; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; max-width: 160px; }
 
     /* ── complete button ── */
     .lesson-complete-wrap {