diff --git a/static/css/app.css b/static/css/app.css
index 5689138..f0eaa59 100644
--- a/static/css/app.css
+++ b/static/css/app.css
@@ -1903,7 +1903,9 @@ body.reader-immersive.reader-show-bottom .reader-overlay  { bottom: var(--bar-h)
 .reader-margin.open .reader-margin-header { display:flex; }
 .reader-margin-title { font-size:11px; text-transform:uppercase; letter-spacing:.04em; color:var(--muted,#888); }
 .reader-margin-header-actions { display:flex; align-items:center; gap:6px; }
-.reader-margin-markers { position:relative; flex:1; overflow-y:auto; overflow-x:hidden; cursor:crosshair; }
+.reader-margin-markers { position:relative; flex:1; overflow:hidden; cursor:crosshair; contain:layout paint; }
+/* The cards in book coordinates; scrolling moves this layer (app.js _repositionMarginMarkers). */
+.margin-layer { position:absolute; top:0; left:0; right:0; height:0; will-change:transform; }
 
 .margin-note { position:absolute; left:8px; right:8px; cursor:pointer; }
 
diff --git a/static/js/app.js b/static/js/app.js
index 1dad5a3..d64eb01 100644
--- a/static/js/app.js
+++ b/static/js/app.js
@@ -5688,7 +5688,7 @@ function resolveAnchorRange(h) {
 
   let range = null;
   try {
-    const blocks = Array.from(contentEl.querySelectorAll(EPUB_BLOCK_SELECTOR));
+    const {blocks} = _epubBlocks(contentEl);
     const startBlock = blocks[h.anchor.startBlockIndex];
     const endBlock = blocks[h.anchor.endBlockIndex] ?? startBlock;
     if (startBlock && endBlock) {
@@ -6061,38 +6061,86 @@ function toggleAnnotationsMargin() {
 // margin — you can read notes there, not just see that one exists); a plain
 // highlight with no note is just a small color dot. Cards are stacked
 // top-to-bottom so overlapping anchors don't overlap visually.
+// Where each highlight sits in the book's scroll coordinates. Finding a range
+// and measuring it is the expensive part, and scrolling doesn't change it:
+// kept until the layout does (content, width, height) or the anchor changes.
+let _marginPosCache = null;
+const _marginCardHeights = new Map(); // `${stripWidth}|${note}` (or 'dot') → px
+
+function _marginDocY(h, contentEl) {
+  const {blocks} = _epubBlocks(contentEl);
+  const key = `${contentEl.clientWidth}x${contentEl.scrollHeight}`;
+  if (!_marginPosCache || _marginPosCache.blocks !== blocks || _marginPosCache.key !== key) {
+    _marginPosCache = {blocks, key, byId: new Map()};
+  }
+  const hit = _marginPosCache.byId.get(h.id);
+  if (hit && hit.anchor === h.anchor) return hit.y;
+  let y = null;
+  try {
+    const range = resolveAnchorRange(h);
+    const rect = range && range.getBoundingClientRect();
+    if (rect && (rect.width || rect.height)) {
+      y = rect.top + rect.height / 2 - contentEl.getBoundingClientRect().top + contentEl.scrollTop;
+    }
+  } catch (e) {}
+  _marginPosCache.byId.set(h.id, {anchor: h.anchor, y});
+  return y;
+}
+
+// The cards live on one layer in the book's coordinates, built when the
+// highlights or the layout change; scrolling only moves the layer (a transform,
+// no layout). Rebuilding and measuring the cards on every scroll frame made a
+// long book with the margin open stutter.
+let _marginLayerKey = null;
+
 function _repositionMarginMarkers() {
   const markersEl = $('reader-margin-markers');
   if (!markersEl) return;
+  if (!readerAnnotationsMarginOpen || currentPdfDoc) {
+    markersEl.innerHTML = '';
+    _marginLayerKey = null;
+    return;
+  }
   if (markersEl.querySelector('.margin-note-editor')) return; // don't blow away an active inline edit
-  markersEl.innerHTML = '';
-  if (!readerAnnotationsMarginOpen || currentPdfDoc) return;
+  const contentEl = $('reader-content');
+  if (!contentEl) return;
 
+  const {blocks} = _epubBlocks(contentEl);
   const areaRect = markersEl.getBoundingClientRect();
-  // A generous pre-filter band: a note card can be considerably taller than
-  // its own anchor point, so a card whose anchor is somewhat outside the
-  // strictly-visible area can still be partially (or fully) on-screen once
-  // rendered. A tight band here excluded it before it ever got a chance to
-  // be measured — the exact bug reported ("tall notes stay hidden until
-  // scrolled much further than expected").
-  const band = Math.max(areaRect.height, 400);
+  const width = Math.round(areaRect.width);
+  const key = `${width}x${contentEl.clientWidth}x${contentEl.scrollHeight}|` +
+    currentHighlights.map(h => `${h.id}:${h.color || ''}:${h.note || ''}`).join('|');
+  let layer = markersEl.querySelector('.margin-layer');
+  if (!layer || key !== _marginLayerKey || !_marginPosCache || _marginPosCache.blocks !== blocks) {
+    markersEl.innerHTML = '';
+    layer = document.createElement('div');
+    layer.className = 'margin-layer';
+    markersEl.appendChild(layer);
+    _renderMarginLayer(layer, contentEl, width);
+    _marginLayerKey = key;
+  }
+  // Book coordinates → this strip's.
+  const shift = contentEl.getBoundingClientRect().top - contentEl.scrollTop - areaRect.top;
+  layer.style.transform = `translateY(${Math.round(shift)}px)`;
+}
+
+// Renders each highlight/note at its text position, in book coordinates.
+// Entries with note text become a readable "post-it" card (the whole point of
+// the margin — you can read notes there, not just see that one exists); a
+// plain highlight with no note is a small colour dot. Cards are stacked top to
+// bottom so overlapping anchors don't overlap visually.
+function _renderMarginLayer(layer, contentEl, width) {
   const entries = [];
   for (const h of currentHighlights) {
-    let range;
-    try { range = resolveAnchorRange(h); } catch (e) { continue; }
-    if (!range) continue;
-    const rect = range.getBoundingClientRect();
-    if (!rect.width && !rect.height) continue; // detached/invalid
-    const y = rect.top + rect.height / 2 - areaRect.top;
-    if (y < -band || y > areaRect.height + band) continue;
-    entries.push({h, y});
+    const y = _marginDocY(h, contentEl);
+    if (y !== null) entries.push({h, y});
   }
   entries.sort((a, b) => a.y - b.y);
 
-  let nextTop = -Infinity;
-  const GAP = 4;
-  const rendered = [];
-  for (const {h, y} of entries) {
+  // All cards first, then their heights in one go (each measured right after
+  // adding it, the browser laid out the whole book once per card). Heights are
+  // kept per note text and strip width.
+  const cards = entries.map(({h, y}) => {
     const hasText = !!h.note;
     const el = document.createElement('div');
     el.className = 'margin-note ' + (hasText ? 'margin-note-text' : 'margin-note-dot');
@@ -6103,22 +6151,19 @@ function _repositionMarginMarkers() {
       el.dataset.color = h.color || 'yellow';
     }
     if (hasText) el.textContent = h.note;
+    layer.appendChild(el);
+    return {el, y, hasText, key: hasText ? `${width}|${h.note}` : 'dot'};
+  });
+  if (_marginCardHeights.size > 500) _marginCardHeights.clear();
+  const unmeasured = cards.filter(c => !_marginCardHeights.has(c.key));
+  for (const c of unmeasured) _marginCardHeights.set(c.key, c.el.getBoundingClientRect().height || (c.hasText ? 20 : 12));
 
-    const desiredTop = hasText ? y - 8 : y - 6;
-    const top = Math.max(desiredTop, nextTop);
+  let nextTop = -Infinity;
+  const GAP = 4;
+  for (const {el, y, hasText, key} of cards) {
+    const top = Math.max(hasText ? y - 8 : y - 6, nextTop);
     el.style.top = top + 'px';
-    markersEl.appendChild(el);
-
-    const measured = el.getBoundingClientRect().height || (hasText ? 20 : 12);
-    nextTop = top + measured + GAP;
-    rendered.push({el, top, measured});
-  }
-
-  // Now that actual card heights are known, do the real cull: drop anything
-  // that ended up entirely outside the visible band after stacking (e.g.
-  // pushed down by a very tall card above it).
-  for (const {el, top, measured} of rendered) {
-    if (top + measured < -40 || top > areaRect.height + 40) el.remove();
+    nextTop = top + _marginCardHeights.get(key) + GAP;
   }
 }
 
@@ -6246,7 +6291,8 @@ function editNoteInlineInMargin(h, pending) {
   let top = pending?.top || 0;
   if (h) {
     const existingCard = markersEl.querySelector(`[data-highlight-id="${h.id}"]`);
-    top = existingCard ? (parseFloat(existingCard.style.top) || 0) : 0;
+    // The card sits on the moving layer: where it shows in the strip right now.
+    top = existingCard ? existingCard.getBoundingClientRect().top - markersEl.getBoundingClientRect().top : 0;
     if (existingCard) existingCard.style.display = 'none';
   }
 
@@ -6261,6 +6307,7 @@ function editNoteInlineInMargin(h, pending) {
   function commit() {
     _commitNoteEdit(h, pending, textarea.value.trim());
     editor.remove();
+    _marginLayerKey = null; // the edited card was hidden: draw the layer anew
     _repositionMarginMarkers();
   }
 
@@ -6277,6 +6324,7 @@ function editNoteInlineInMargin(h, pending) {
       e.stopPropagation();
       textarea.removeEventListener('blur', commit); // discard, don't save
       editor.remove();
+      _marginLayerKey = null;
       _repositionMarginMarkers();
     }
   });
diff --git a/static/js/sw.js b/static/js/sw.js
index 467ebc3..cc0af47 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-v103';
+const CACHE = 'diora-v104';
 const SHELL = [
   '/static/css/app.css',
   '/static/js/app.js',