diff --git a/static/js/app.js b/static/js/app.js
index fe12f31..a094a04 100644
--- a/static/js/app.js
+++ b/static/js/app.js
@@ -2120,26 +2120,88 @@ function _checkScrollJump(contentEl) {
 
 const EPUB_BLOCK_SELECTOR = 'p, h1, h2, h3, h4, h5, h6, li, blockquote, dt, dd, figcaption, div:not(:has(*))';
 
-function getPositionAnchor(contentEl) {
+// The open book's text blocks (EPUB_BLOCK_SELECTOR), with the chapter each one
+// belongs to — found once and kept until the book's content changes (a
+// highlight wrapping text can turn a leaf div into a non-leaf one). Selecting
+// them is the expensive part on an e-reader: Chrome 61 has no :has(), so
+// compat61.js filters every div in JavaScript.
+let _blockCache = null; // {content, blocks, chapterOf: Int32Array, chapters: Element[]}
+
+function _epubBlocks(contentEl) {
+  if (_blockCache && _blockCache.content === contentEl) return _blockCache;
   const blocks = Array.from(contentEl.querySelectorAll(EPUB_BLOCK_SELECTOR));
+  const chapters = Array.from(contentEl.querySelectorAll('[data-epub-src]'));
+  const chapterOf = new Int32Array(blocks.length);
+  for (let i = 0, ch = -1; i < blocks.length; i++) {
+    while (ch + 1 < chapters.length && chapters[ch + 1].compareDocumentPosition(blocks[i]) & Node.DOCUMENT_POSITION_FOLLOWING) ch++;
+    chapterOf[i] = ch;
+  }
+  const observer = new MutationObserver(() => {
+    if (_blockCache && _blockCache.content === contentEl) _blockCache = null;
+    observer.disconnect();
+  });
+  observer.observe(contentEl, {childList: true, subtree: true});
+  _blockCache = {content: contentEl, blocks, chapterOf, chapters};
+  return _blockCache;
+}
+
+// The anchor of the reading position: "block:inner" — the last block (with a
+// height) that starts at or above the top of the view, the first of several
+// starting at the same height; else the first block below. Blocks are in
+// document order and so top to bottom: a binary search over the chapters on
+// screen (the e-reader mode shows one), not every block from the book's start.
+function getPositionAnchor(contentEl) {
+  const {blocks, chapterOf, chapters} = _epubBlocks(contentEl);
   if (!blocks.length) return '';
   const containerTop = contentEl.getBoundingClientRect().top;
   const containerBottom = containerTop + contentEl.clientHeight;
-  let bestIndex = 0;
-  let bestDelta = Infinity;
+  // Index ranges of the blocks in chapters that are laid out (all, when reading by scrolling).
+  const shown = new Set();
+  chapters.forEach((ch, i) => { if (ch.getClientRects().length) shown.add(i); });
+  const order = [];
   for (let i = 0; i < blocks.length; i++) {
-    const rect = blocks[i].getBoundingClientRect();
-    if (rect.height < 1) continue;
-    if (rect.top > containerBottom) break;
-    const delta = rect.top - containerTop;
-    if (delta <= 0 && Math.abs(delta) < Math.abs(bestDelta)) {
-      bestIndex = i;
-      bestDelta = delta;
-    } else if (delta > 0 && bestDelta === Infinity) {
-      bestIndex = i;
-      bestDelta = delta;
+    if (chapterOf[i] < 0 || shown.has(chapterOf[i])) {
+      let j = i;
+      while (j + 1 < blocks.length && chapterOf[j + 1] === chapterOf[i]) j++;
+      order.push([i, j]);
+      i = j;
     }
   }
+  const count = order.reduce((n, r) => n + r[1] - r[0] + 1, 0);
+  const at = k => { // k-th block on screen -> block index
+    for (const [a, b] of order) { if (k <= b - a) return a + k; k -= b - a + 1; }
+    return -1;
+  };
+  const rectOf = k => blocks[at(k)].getBoundingClientRect();
+  // A block without height says nothing about the position: use the next one with a height.
+  const measured = k => {
+    for (let n = k; n < count; n++) { const r = rectOf(n); if (r.height >= 1) return {k: n, top: r.top}; }
+    return null;
+  };
+  // Binary search for the last block with a height whose top <= containerTop.
+  let lo = 0, hi = count - 1, best = -1;
+  while (lo <= hi) {
+    const mid = (lo + hi) >> 1;
+    const m = measured(mid);
+    if (m && m.top <= containerTop) { best = m.k; lo = m.k + 1; }
+    else hi = mid - 1;
+  }
+  let bestIndex;
+  if (best >= 0) {
+    // Several blocks starting at the same height (a blockquote and its first paragraph): the first.
+    const top = rectOf(best).top;
+    let k = best;
+    for (let n = best - 1; n >= 0; n--) {
+      const r = rectOf(n);
+      if (r.height < 1) continue;
+      if (r.top !== top) break;
+      k = n;
+    }
+    bestIndex = at(k);
+  } else {
+    const first = measured(0);
+    bestIndex = first && first.top <= containerBottom ? at(first.k) : 0;
+  }
   const rect = blocks[bestIndex].getBoundingClientRect();
   const innerFraction = Math.max(0, Math.min(1, (containerTop - rect.top) / (rect.height || 1)));
   return `${bestIndex}:${innerFraction.toFixed(6)}`;
diff --git a/static/js/sw.js b/static/js/sw.js
index fba4890..853f5fb 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-v97';
+const CACHE = 'diora-v98';
 const SHELL = [
   '/static/css/app.css',
   '/static/js/app.js',