diff --git a/books/views.py b/books/views.py
index a36797f..3a7ba80 100644
--- a/books/views.py
+++ b/books/views.py
@@ -16,6 +16,30 @@ def _require_auth(request):
     return None
 
 
+def _anchor_parts(anchor):
+    """Split a position anchor 'blockIndex:innerFraction' into (block, inner).
+
+    Returns (-1, 0.0) for empty/invalid anchors (e.g. PDF progress)."""
+    if not isinstance(anchor, str) or ':' not in anchor:
+        return (-1, 0.0)
+    block, _, inner = anchor.partition(':')
+    try:
+        return (int(block), float(inner))
+    except ValueError:
+        return (-1, 0.0)
+
+
+def _progress_is_further(new_anchor, new_frac, old_anchor, old_frac):
+    """True if the new reading position is at least as far into the book as the
+    old one. Compares by anchor block index (precise, decoupled from layout);
+    falls back to scroll_fraction only when an anchor is missing (PDFs)."""
+    nb, ni = _anchor_parts(new_anchor)
+    ob, oi = _anchor_parts(old_anchor)
+    if nb >= 0 and ob >= 0:
+        return ni >= oi if nb == ob else nb >= ob
+    return new_frac >= old_frac
+
+
 @require_http_methods(['GET'])
 def book_list(request):
     err = _require_auth(request)
@@ -210,15 +234,13 @@ def save_progress(request, pk):
         user=request.user,
         book=book,
     )
-    # Always advance to the furthest-read position unless the client
-    # explicitly forces a reset (e.g. "start over" button in the reader).
-    if force:
+    # Always advance to the furthest-read position (by anchor block index, so a
+    # transiently wrong scroll_fraction can't freeze the position) unless the
+    # client explicitly forces a reset (e.g. "start over" button in the reader).
+    if force or _progress_is_further(position_anchor, scroll_fraction,
+                                     progress.position_anchor, progress.scroll_fraction):
         progress.scroll_fraction = scroll_fraction
         progress.position_anchor = position_anchor
-    else:
-        if scroll_fraction >= progress.scroll_fraction:
-            progress.scroll_fraction = scroll_fraction
-            progress.position_anchor = position_anchor
     progress.save(update_fields=['scroll_fraction', 'position_anchor', 'updated_at'])
 
     return JsonResponse({'ok': True})
diff --git a/static/js/app.js b/static/js/app.js
index 95d13cc..c4872b2 100644
--- a/static/js/app.js
+++ b/static/js/app.js
@@ -2699,6 +2699,26 @@ function restoreFromAnchor(contentEl, anchor) {
   return true;
 }
 
+// Split 'blockIndex:innerFraction' into [block, inner]; [-1, 0] if invalid/empty.
+function _anchorParts(anchor) {
+  if (typeof anchor !== 'string') return [-1, 0];
+  const c = anchor.indexOf(':');
+  if (c < 0) return [-1, 0];
+  const b = parseInt(anchor.slice(0, c), 10);
+  const f = parseFloat(anchor.slice(c + 1));
+  return [Number.isFinite(b) ? b : -1, Number.isFinite(f) ? f : 0];
+}
+
+// >0 if position A is further into the book than B, <0 if behind, 0 if equal.
+// Compares by anchor block index (precise); falls back to scroll_fraction only
+// when an anchor is missing (e.g. PDFs), matching the server's _progress_is_further.
+function _cmpProgress(anchorA, fracA, anchorB, fracB) {
+  const [ba, ia] = _anchorParts(anchorA);
+  const [bb, ib] = _anchorParts(anchorB);
+  if (ba >= 0 && bb >= 0) return ba !== bb ? ba - bb : ia - ib;
+  return (fracA || 0) - (fracB || 0);
+}
+
 // ---------------------------------------------------------------------------
 // Book cache — IndexedDB, evict after 4 weeks of inactivity
 // ---------------------------------------------------------------------------
@@ -2815,7 +2835,7 @@ async function _loadCachedBookMeta() {
   } catch { return []; }
 }
 
-async function _updateCachedBookProgress(bookId, fraction, anchor) {
+async function _updateCachedBookProgress(bookId, fraction, anchor, force = false) {
   try {
     const db = await _openBookCacheDb();
     await new Promise(resolve => {
@@ -2824,7 +2844,8 @@ async function _updateCachedBookProgress(bookId, fraction, anchor) {
       const req = store.get(bookId);
       req.onsuccess = e => {
         const entry = e.target.result;
-        if (entry) {
+        // Only advance (or on force), so a transiently wrong save can't lose progress.
+        if (entry && (force || _cmpProgress(anchor, fraction, entry.position_anchor, entry.scroll_fraction) >= 0)) {
           entry.scroll_fraction  = fraction;
           entry.position_anchor  = anchor;
           entry.last_read        = new Date().toISOString();
@@ -2988,7 +3009,7 @@ async function loadBookList() {
     const _localById = Object.fromEntries(_localMeta.map(b => [b.id, b]));
     for (const b of decrypted) {
       const loc = _localById[b.id];
-      if (loc && loc.scroll_fraction > b.scroll_fraction) {
+      if (loc && _cmpProgress(loc.position_anchor, loc.scroll_fraction, b.position_anchor, b.scroll_fraction) > 0) {
         fetch(`/books/${b.id}/progress/`, {
           method: 'POST',
           headers: {'Content-Type': 'application/json', 'X-CSRFToken': getCsrfToken()},
@@ -3731,7 +3752,7 @@ async function saveReaderProgress(force = false) {
     _currentPositionAnchor = anchor;
   }
 
-  _updateCachedBookProgress(currentBookId, fraction, anchor);
+  _updateCachedBookProgress(currentBookId, fraction, anchor, force);
 
   const body = JSON.stringify({scroll_fraction: fraction, position_anchor: anchor, ...(force && {force: true})});
   _lastProgressBeacon = {url: `/books/${currentBookId}/progress/`, body};
diff --git a/static/js/sw.js b/static/js/sw.js
index 2860d89..8f11f96 100644
--- a/static/js/sw.js
+++ b/static/js/sw.js
@@ -2,7 +2,7 @@
  * diora service worker — caches the app shell for offline use.
  */
 
-const CACHE = 'diora-v14';
+const CACHE = 'diora-v15';
 const PODCAST_CACHE = 'diora-podcast-v1';
 const SHELL = [
   '/static/css/app.css',