diff --git a/app/index.html b/app/index.html
index fc751c2..8bba01b 100644
--- a/app/index.html
+++ b/app/index.html
@@ -5,7 +5,7 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>OverBoard PDF</title>
 <meta name="description" content="OverBoard: write on any PDF on your smartboard. Runs in the browser; nothing is uploaded.">
-<meta name="ob-version" content="4d7b27241fc6">
+<meta name="ob-version" content="86f90347201b">
 <meta name="theme-color" content="#1f1f1f">
 <link rel="manifest" href="manifest.json">
 <link rel="icon" href="icons/icon-192.png">
@@ -45,7 +45,7 @@ <h2 data-t="open_again">Open again</h2>
     <p class="note"><span data-th="start_note">Your PDF and your ink stay in this browser on this computer. Nothing is uploaded.
       Have a Word file? In Word, choose File → Save as PDF, then drop the PDF here.
       Have an HTML worksheet? Drop it here to get a <i>_board.html</i> copy you can write on.</span>
-      <br><a href="../" data-t="back_demo">Back to the HTML worksheet demo</a> · <a href="https://github.com/BPMData/OverBoard" data-t="about">About OverBoard</a> · <a href="report/?from=start" target="_blank" data-t="board_help">Board not working?</a> · <span id="ob-release" title="OverBoard version">v30</span></p>
+      <br><a href="../" data-t="back_demo">Back to the HTML worksheet demo</a> · <a href="https://github.com/BPMData/OverBoard" data-t="about">About OverBoard</a> · <a href="report/?from=start" target="_blank" data-t="board_help">Board not working?</a> · <span id="ob-release" title="OverBoard version">v31</span></p>
     <p class="note ob-langpick"><label><span aria-hidden="true">🌐</span> <select id="ob-lang" aria-label="Language" data-ta="language" data-tt="language" title="Language"></select></label></p>
   </div>
 </main>
diff --git a/app/report/index.html b/app/report/index.html
index 2b06b92..6fe8a44 100644
--- a/app/report/index.html
+++ b/app/report/index.html
@@ -96,7 +96,7 @@ <h1 data-t="r_h1">Is OverBoard not working on your board?</h1>
     <p data-th="r_intro2">If something doesn't work the way the page asks, that's fine. <b>That's exactly what I need to know.</b> Just keep going.</p>
     <p class="mute small" data-th="r_privacy">Nothing is sent until you press <b>Send</b> at the very end, and you'll see everything first. No worksheets, PDFs or ink are ever included.</p>
     <div class="row"><button class="go big" data-go="start" data-t="r_start">Start</button></div>
-    <p class="mute small" style="margin-top:28px">OverBoard <span id="ob-release" title="OverBoard version">v30</span></p>
+    <p class="mute small" style="margin-top:28px">OverBoard <span id="ob-release" title="OverBoard version">v31</span></p>
   </div></div>
 </section>
 
diff --git a/app/sw.js b/app/sw.js
index d7e2a1c..ec13a80 100644
--- a/app/sw.js
+++ b/app/sw.js
@@ -1,7 +1,7 @@
 // ===== GENERATED by tools/build.py: every file the app needs, with the start of its SHA-256. Do not edit by hand. =====
-const VERSION = 'e1d36925f338';
+const VERSION = '02572123a8ee';
 const FILES = {
-  "../overlay/ink_overlay.html": "31eb701f9f240009",
+  "../overlay/ink_overlay.html": "c8af57d0d48b19cd",
   "../shared/ink-core.js": "f5235bb4ef99a375",
   "../shared/strings.js": "d0525093455e4d45",
   "../shared/toolbar.css": "d3155c02c903c96e",
@@ -14,14 +14,14 @@ const FILES = {
   "icons/icon-192.png": "d245e15b36475003",
   "icons/icon-512.png": "17c1021fe2a0ade1",
   "icons/maskable-512.png": "0c58f1bdccc9eda9",
-  "index.html": "18ce16d379756213",
+  "index.html": "4b5c66216bb59125",
   "ink.js": "069771a403be1378",
   "libs.js": "c09f60186ab9029a",
   "main.js": "ac823fa04ab55eb2",
   "manifest.json": "f7ee4a6c6f83cd40",
   "notebook.js": "47bcfd4f80712344",
   "offline.js": "6504935b2738711a",
-  "report/index.html": "084e1351caafa212",
+  "report/index.html": "beee65975d0d2445",
   "store.js": "66af54cd32e6cbd2",
   "textlayer.js": "1911308ed3e2a8b5",
   "vendor/pdf-lib@1.17.1/LICENSE.md": "f2c9fc00fdb66eb9",
@@ -214,7 +214,7 @@ const FILES = {
   "vendor/pdfjs-dist@4.10.38/standard_fonts/LiberationSans-BoldItalic.ttf": "a224075ac17495ad",
   "vendor/pdfjs-dist@4.10.38/standard_fonts/LiberationSans-Italic.ttf": "832b4406dbef2362",
   "vendor/pdfjs-dist@4.10.38/standard_fonts/LiberationSans-Regular.ttf": "f8ace1f892b2bd9d",
-  "version.json": "445c090471c65004",
+  "version.json": "f59b4daff279aa28",
   "viewer.js": "648b4479c5b68b93"
 };
 // ===== END GENERATED =====
diff --git a/app/version.json b/app/version.json
index 202cad3..0c547a7 100644
--- a/app/version.json
+++ b/app/version.json
@@ -1,5 +1,5 @@
 {
-  "v": "4d7b27241fc6",
+  "v": "86f90347201b",
   "js": [
     "embed.js",
     "export.js",
diff --git a/index.html b/index.html
index 82c4974..d837759 100644
--- a/index.html
+++ b/index.html
@@ -54,7 +54,7 @@ <h2>Exit ticket</h2>
   <div class="lines"><div></div><div></div><div></div><div></div></div>
   <div class="foot">OverBoard · page 2 / 2 · Your ink saves in this browser only. Nothing is uploaded.</div>
 </div>
-<!-- ===== INK OVERLAY START (v30) =====
+<!-- ===== INK OVERLAY START (v31) =====
   Smartboard writing layer. Paste this whole block just before </body> in any .html file.
   SMART BOARD: pen tip writes. Palm, fist or side of hand erases (the board reports it as an eraser).
   Finger (and the round end of the SMART pen) scrolls the page; the "Finger" button switches it to drawing if you lose the pen.
@@ -1686,14 +1686,20 @@ <h2>Exit ticket</h2>
   /* ---------- drawing ---------- */
   // pen ink and covers go on the top canvas, highlights on the layer under it, the eraser cuts through both
   function layers(s){ return s.e ? [ctx, hctx] : seeThrough(s) ? [hctx] : [ctx]; }
-  function drawStroke(s){ layers(s).forEach(function(g){ paintStroke(g, s, scale, OX * scale, 0); }); }
+  function drawStroke(s){ layers(s).forEach(function(g){ paintStroke(g, s, scale, OX * scale, -BY * scale); }); }
   // live drawing draws the same curve piece by piece as points arrive (matches paintStroke exactly)
   var live = null;
-  function drawPieces(s, pieces){ layers(s).forEach(function(g){ paintPieces(g, s, pieces, scale, OX * scale, 0); }); }
+  function drawPieces(s, pieces){ layers(s).forEach(function(g){ paintPieces(g, s, pieces, scale, OX * scale, -BY * scale); }); }
   function clearCanvas(c, g){ g.save(); g.setTransform(1,0,0,1,0,0); g.clearRect(0,0,c.width,c.height); g.restore(); }
+  function inBand(s){   // skip strokes wholly above or below the canvas band
+    var lo = Infinity, hi = -Infinity, p = s.p;
+    for (var k = 1; k < p.length; k += 2) { if (p[k] < lo) lo = p[k]; if (p[k] > hi) hi = p[k]; }
+    return hi + s.w >= BY && lo - s.w <= BY + BH;
+  }
   function redraw(){
     clearCanvas(canvas, ctx); clearCanvas(hcan, hctx);
-    for (var i = 0; i < strokes.length; i++) drawStroke(strokes[i]);
+    for (var i = 0; i < strokes.length; i++) if (inBand(strokes[i])) drawStroke(strokes[i]);
+    if (cur) drawStroke(cur);   // a stroke still being drawn when the band moves
   }
   function calcCW(){
     var w = 0;
@@ -1702,27 +1708,44 @@ <h2>Exit ticket</h2>
   }
   // The canvas spans the whole window so you can write in the empty space beside the page.
   // Ink is stored relative to the page column (CW wide, centered), so it stays lined up when the window or zoom changes.
-  var FW = 0, OX = 0;   // full canvas width (CSS px) and where the page column starts inside it
+  // Up and down, the canvas is only a band about three screens tall that follows the scrolling (BY = its top, BH = its
+  // height, in worksheet px). A canvas taller than 16384 screen pixels can't be drawn by the graphics chip, so Chrome
+  // falls back to the processor and every pen move gets slow: a 20-page worksheet on a 2x (Retina) screen ran at a few
+  // frames a second.
+  var FW = 0, OX = 0, BY = 0, BH = 0;   // full canvas width (CSS px) and where the page column starts inside it
+  function docTop(){ return canvas.getBoundingClientRect().top - BY * Z; }   // screen y of the worksheet's top edge
   function size(){
     var h = Math.ceil(body.offsetHeight), cw = calcCW();
     var fw = Math.max(cw, Math.floor(document.documentElement.clientWidth / Z));
-    var sc = Math.min((window.devicePixelRatio || 1) * Z, 2 * Math.max(1, Z), 30000 / h, 30000 / fw, Math.sqrt(100e6 / (fw * h)));   // sharp when zoomed in, under browser canvas limits (two canvases)
-    if (h === H && sc === scale && cw === CW && fw === FW && canvas.width) return;
-    H = h; scale = sc; CW = cw; FW = fw; OX = (fw - cw) / 2;
+    var bh = Math.min(h, Math.ceil(3 * innerHeight / Z));
+    var sc = Math.min((window.devicePixelRatio || 1) * Z, 2 * Math.max(1, Z), 16000 / bh, 16000 / fw, Math.sqrt(100e6 / (fw * bh)));   // sharp when zoomed in, small enough for the graphics chip
+    if (h === H && sc === scale && cw === CW && fw === FW && bh === BH && canvas.width) { follow(); return; }
+    H = h; scale = sc; CW = cw; FW = fw; OX = (fw - cw) / 2; BH = bh;
     [canvas, hcan].forEach(function(c){
-      c.width = Math.round(FW * scale); c.height = Math.round(H * scale);
-      c.style.width = FW + 'px'; c.style.height = H + 'px';
+      c.width = Math.round(FW * scale); c.height = Math.round(BH * scale);
+      c.style.width = FW + 'px'; c.style.height = BH + 'px';
     });
+    follow(true);
+  }
+  // keep the band around what's on screen, with a screen to spare above and below; redraw when it moves
+  function follow(force){
+    var vh = innerHeight / Z, top = -docTop() / Z, by = BY;
+    if (force || top < BY + (BY > 0 ? vh / 3 : 0) || top + vh > BY + BH - (BY + BH < H ? vh / 3 : 0))
+      by = Math.round(Math.max(0, Math.min(H - BH, top - (BH - vh) / 2)));
+    if (by === BY && !force) return;
+    BY = by;
+    canvas.style.top = hcan.style.top = BY + 'px';
     redraw();
   }
 
   function pt(e){
-    var r = canvas.getBoundingClientRect();   // screen px; ink is in the worksheet's own px, so divide by the zoom
-    return [Math.round(((e.clientX - r.left) / Z - OX) * 10) / 10, Math.round((e.clientY - r.top) / Z * 10) / 10];
+    var t = docTop(), r = canvas.getBoundingClientRect();   // screen px; ink is in the worksheet's own px, so divide by the zoom
+    return [Math.round(((e.clientX - r.left) / Z - OX) * 10) / 10, Math.round((e.clientY - t) / Z * 10) / 10];
   }
   canvas.addEventListener('pointerdown', function(e){
     if (active !== null || !ready) return;
     if (e.pointerType === 'mouse' && e.button !== 0) return;
+    follow();
     // SMART board: pen tip = "pen"; palm/fist/side of hand = "pen" with the eraser button (32); finger and round pen end = "mouse"
     var kind = pointerKind(e), palm = kind === 'palm', finger = kind === 'finger';
     active = e.pointerId;
@@ -1783,7 +1806,7 @@ <h2>Exit ticket</h2>
      it count as that page); "this page" is the one at the middle of the screen */
   function pages(){ return [].slice.call(document.querySelectorAll('.page')); }
   function clearPage(){
-    var ct = canvas.getBoundingClientRect().top;
+    var ct = docTop();
     var tops = pages().map(function(pg){ return (pg.getBoundingClientRect().top - ct) / Z; });
     function band(y){ var i = 0; while (i + 1 < tops.length && tops[i + 1] <= y) i++; return i; }
     var n = band((innerHeight / 2 - ct) / Z);
@@ -2144,7 +2167,7 @@ <h2>Exit ticket</h2>
     clearPrint();
     var list = [].slice.call(document.querySelectorAll('.page'));
     if (!list.length) list = [body];
-    var ctop = canvas.getBoundingClientRect().top, hasHl = !PRINT_INK_BLACK && strokes.some(seeThrough);
+    var ctop = docTop(), hasHl = !PRINT_INK_BLACK && strokes.some(seeThrough);
     list.forEach(function(pg){
       var ph = pg.offsetHeight, pw = pg.offsetWidth, top = (pg === body) ? 0 : pg.getBoundingClientRect().top - ctop;
       var ps = Math.min(PRINT_SCALE, 30000 / ph, Math.sqrt(60e6 / (CW * ph)));
@@ -2171,6 +2194,7 @@ <h2>Exit ticket</h2>
   /* ---------- start ---------- */
   var src = load(); size(); translateAll(); renderPeriods(); refresh(); setMode('write'); layoutClear(); fitToolbar();
   window.addEventListener('resize', size);
+  window.addEventListener('scroll', function(){ follow(); }, { passive: true });
   window.addEventListener('load', size);
   if (window.ResizeObserver) new ResizeObserver(size).observe(body);
   function started(src){ setStatus(tr(src === 'file' ? 'st_loaded_file' : src === 'browser' ? 'st_restored' : src === 'old' ? 'st_old_carried' : 'st_ready', { p: D.active })); }
diff --git a/overlay/ink_overlay.html b/overlay/ink_overlay.html
index 19835c2..b37cad9 100644
--- a/overlay/ink_overlay.html
+++ b/overlay/ink_overlay.html
@@ -1,4 +1,4 @@
-<!-- ===== INK OVERLAY START (v30) =====
+<!-- ===== INK OVERLAY START (v31) =====
   Smartboard writing layer. Paste this whole block just before </body> in any .html file.
   SMART BOARD: pen tip writes. Palm, fist or side of hand erases (the board reports it as an eraser).
   Finger (and the round end of the SMART pen) scrolls the page; the "Finger" button switches it to drawing if you lose the pen.
@@ -1630,14 +1630,20 @@
   /* ---------- drawing ---------- */
   // pen ink and covers go on the top canvas, highlights on the layer under it, the eraser cuts through both
   function layers(s){ return s.e ? [ctx, hctx] : seeThrough(s) ? [hctx] : [ctx]; }
-  function drawStroke(s){ layers(s).forEach(function(g){ paintStroke(g, s, scale, OX * scale, 0); }); }
+  function drawStroke(s){ layers(s).forEach(function(g){ paintStroke(g, s, scale, OX * scale, -BY * scale); }); }
   // live drawing draws the same curve piece by piece as points arrive (matches paintStroke exactly)
   var live = null;
-  function drawPieces(s, pieces){ layers(s).forEach(function(g){ paintPieces(g, s, pieces, scale, OX * scale, 0); }); }
+  function drawPieces(s, pieces){ layers(s).forEach(function(g){ paintPieces(g, s, pieces, scale, OX * scale, -BY * scale); }); }
   function clearCanvas(c, g){ g.save(); g.setTransform(1,0,0,1,0,0); g.clearRect(0,0,c.width,c.height); g.restore(); }
+  function inBand(s){   // skip strokes wholly above or below the canvas band
+    var lo = Infinity, hi = -Infinity, p = s.p;
+    for (var k = 1; k < p.length; k += 2) { if (p[k] < lo) lo = p[k]; if (p[k] > hi) hi = p[k]; }
+    return hi + s.w >= BY && lo - s.w <= BY + BH;
+  }
   function redraw(){
     clearCanvas(canvas, ctx); clearCanvas(hcan, hctx);
-    for (var i = 0; i < strokes.length; i++) drawStroke(strokes[i]);
+    for (var i = 0; i < strokes.length; i++) if (inBand(strokes[i])) drawStroke(strokes[i]);
+    if (cur) drawStroke(cur);   // a stroke still being drawn when the band moves
   }
   function calcCW(){
     var w = 0;
@@ -1646,27 +1652,44 @@
   }
   // The canvas spans the whole window so you can write in the empty space beside the page.
   // Ink is stored relative to the page column (CW wide, centered), so it stays lined up when the window or zoom changes.
-  var FW = 0, OX = 0;   // full canvas width (CSS px) and where the page column starts inside it
+  // Up and down, the canvas is only a band about three screens tall that follows the scrolling (BY = its top, BH = its
+  // height, in worksheet px). A canvas taller than 16384 screen pixels can't be drawn by the graphics chip, so Chrome
+  // falls back to the processor and every pen move gets slow: a 20-page worksheet on a 2x (Retina) screen ran at a few
+  // frames a second.
+  var FW = 0, OX = 0, BY = 0, BH = 0;   // full canvas width (CSS px) and where the page column starts inside it
+  function docTop(){ return canvas.getBoundingClientRect().top - BY * Z; }   // screen y of the worksheet's top edge
   function size(){
     var h = Math.ceil(body.offsetHeight), cw = calcCW();
     var fw = Math.max(cw, Math.floor(document.documentElement.clientWidth / Z));
-    var sc = Math.min((window.devicePixelRatio || 1) * Z, 2 * Math.max(1, Z), 30000 / h, 30000 / fw, Math.sqrt(100e6 / (fw * h)));   // sharp when zoomed in, under browser canvas limits (two canvases)
-    if (h === H && sc === scale && cw === CW && fw === FW && canvas.width) return;
-    H = h; scale = sc; CW = cw; FW = fw; OX = (fw - cw) / 2;
+    var bh = Math.min(h, Math.ceil(3 * innerHeight / Z));
+    var sc = Math.min((window.devicePixelRatio || 1) * Z, 2 * Math.max(1, Z), 16000 / bh, 16000 / fw, Math.sqrt(100e6 / (fw * bh)));   // sharp when zoomed in, small enough for the graphics chip
+    if (h === H && sc === scale && cw === CW && fw === FW && bh === BH && canvas.width) { follow(); return; }
+    H = h; scale = sc; CW = cw; FW = fw; OX = (fw - cw) / 2; BH = bh;
     [canvas, hcan].forEach(function(c){
-      c.width = Math.round(FW * scale); c.height = Math.round(H * scale);
-      c.style.width = FW + 'px'; c.style.height = H + 'px';
+      c.width = Math.round(FW * scale); c.height = Math.round(BH * scale);
+      c.style.width = FW + 'px'; c.style.height = BH + 'px';
     });
+    follow(true);
+  }
+  // keep the band around what's on screen, with a screen to spare above and below; redraw when it moves
+  function follow(force){
+    var vh = innerHeight / Z, top = -docTop() / Z, by = BY;
+    if (force || top < BY + (BY > 0 ? vh / 3 : 0) || top + vh > BY + BH - (BY + BH < H ? vh / 3 : 0))
+      by = Math.round(Math.max(0, Math.min(H - BH, top - (BH - vh) / 2)));
+    if (by === BY && !force) return;
+    BY = by;
+    canvas.style.top = hcan.style.top = BY + 'px';
     redraw();
   }
 
   function pt(e){
-    var r = canvas.getBoundingClientRect();   // screen px; ink is in the worksheet's own px, so divide by the zoom
-    return [Math.round(((e.clientX - r.left) / Z - OX) * 10) / 10, Math.round((e.clientY - r.top) / Z * 10) / 10];
+    var t = docTop(), r = canvas.getBoundingClientRect();   // screen px; ink is in the worksheet's own px, so divide by the zoom
+    return [Math.round(((e.clientX - r.left) / Z - OX) * 10) / 10, Math.round((e.clientY - t) / Z * 10) / 10];
   }
   canvas.addEventListener('pointerdown', function(e){
     if (active !== null || !ready) return;
     if (e.pointerType === 'mouse' && e.button !== 0) return;
+    follow();
     // SMART board: pen tip = "pen"; palm/fist/side of hand = "pen" with the eraser button (32); finger and round pen end = "mouse"
     var kind = pointerKind(e), palm = kind === 'palm', finger = kind === 'finger';
     active = e.pointerId;
@@ -1727,7 +1750,7 @@
      it count as that page); "this page" is the one at the middle of the screen */
   function pages(){ return [].slice.call(document.querySelectorAll('.page')); }
   function clearPage(){
-    var ct = canvas.getBoundingClientRect().top;
+    var ct = docTop();
     var tops = pages().map(function(pg){ return (pg.getBoundingClientRect().top - ct) / Z; });
     function band(y){ var i = 0; while (i + 1 < tops.length && tops[i + 1] <= y) i++; return i; }
     var n = band((innerHeight / 2 - ct) / Z);
@@ -2088,7 +2111,7 @@
     clearPrint();
     var list = [].slice.call(document.querySelectorAll('.page'));
     if (!list.length) list = [body];
-    var ctop = canvas.getBoundingClientRect().top, hasHl = !PRINT_INK_BLACK && strokes.some(seeThrough);
+    var ctop = docTop(), hasHl = !PRINT_INK_BLACK && strokes.some(seeThrough);
     list.forEach(function(pg){
       var ph = pg.offsetHeight, pw = pg.offsetWidth, top = (pg === body) ? 0 : pg.getBoundingClientRect().top - ctop;
       var ps = Math.min(PRINT_SCALE, 30000 / ph, Math.sqrt(60e6 / (CW * ph)));
@@ -2115,6 +2138,7 @@
   /* ---------- start ---------- */
   var src = load(); size(); translateAll(); renderPeriods(); refresh(); setMode('write'); layoutClear(); fitToolbar();
   window.addEventListener('resize', size);
+  window.addEventListener('scroll', function(){ follow(); }, { passive: true });
   window.addEventListener('load', size);
   if (window.ResizeObserver) new ResizeObserver(size).observe(body);
   function started(src){ setStatus(tr(src === 'file' ? 'st_loaded_file' : src === 'browser' ? 'st_restored' : src === 'old' ? 'st_old_carried' : 'st_ready', { p: D.active })); }