Render PDF from current page outward on zoom/re-render

renderPdf now takes an optional pivotPage. Pass 1 builds the full DOM
and sizes all canvases (instant). Then scrolls to the pivot page
immediately so the user stays in place. Pass 2 renders pixels from
pivot→end, then pivot-1→start.

reRenderPdf detects the current visible page before re-rendering and
passes it as pivot, so zoom no longer resets scroll position.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/js/app.js

@@ -3010,7 +3010,7 @@ async function _parsePdfOutline(pdf, items, depth) {
   return result;
 }
 
-async function renderPdf(arrayBuffer, contentEl, scaleOverride) {
+async function renderPdf(arrayBuffer, contentEl, scaleOverride, pivotPage) {
   const myGen = ++_pdfRenderGen;
   const pdf = currentPdfDoc || await pdfjsLib.getDocument({data: new Uint8Array(arrayBuffer.slice(0))}).promise;
   if (_pdfRenderGen !== myGen) return null;
@@ -3031,7 +3031,6 @@ async function renderPdf(arrayBuffer, contentEl, scaleOverride) {
 
   contentEl.innerHTML = '';
 
-  // Viewport wrapper: CSS zoom controls display scale without re-rendering
   const pdfVp = document.createElement('div');
   pdfVp.id = 'pdf-viewport';
   contentEl.appendChild(pdfVp);
@@ -3039,15 +3038,17 @@ async function renderPdf(arrayBuffer, contentEl, scaleOverride) {
   const containerWidth = readerSettings.pdfSpread
     ? contentEl.clientWidth - 32
     : Math.min(contentEl.clientWidth - 32, 900);
+  const pageWidth = readerSettings.pdfSpread ? (containerWidth - 8) / 2 : containerWidth;
+  const dpr = window.devicePixelRatio || 1;
 
+  // Pass 1: build DOM structure and size all canvases (no pixel rendering yet)
+  const pageEntries = [];
   let spreadContainer = null;
 
   for (let pageNum = 1; pageNum <= pdf.numPages; pageNum++) {
     if (_pdfRenderGen !== myGen) { contentEl.innerHTML = ''; return null; }
     const page = await pdf.getPage(pageNum);
     const naturalVp = page.getViewport({scale: 1});
-    const pageWidth = readerSettings.pdfSpread ? (containerWidth - 8) / 2 : containerWidth;
-    // Zoom baked into canvas resolution — no CSS zoom, stays sharp at any DPR
     const scale = scaleOverride != null ? scaleOverride
       : Math.max(0.5, pageWidth / naturalVp.width) * (readerSettings.pdfZoom / 100);
     const viewport = page.getViewport({scale});
@@ -3056,12 +3057,9 @@ async function renderPdf(arrayBuffer, contentEl, scaleOverride) {
     wrapper.className = 'pdf-page-wrapper';
     wrapper.id = `pdf-page-${pageNum}`;
 
-    // Inner container gives canvas + text layer a shared position:relative origin,
-    // independent of the outer flex wrapper's centering.
     const inner = document.createElement('div');
     inner.className = 'pdf-page-inner';
 
-    const dpr = window.devicePixelRatio || 1;
     const canvas = document.createElement('canvas');
     canvas.className = 'pdf-page';
     canvas.width = Math.round(viewport.width * dpr);
@@ -3087,14 +3085,29 @@ async function renderPdf(arrayBuffer, contentEl, scaleOverride) {
       else pdfVp.appendChild(wrapper);
     }
 
-    const ctx = canvas.getContext('2d');
+    pageEntries.push({page, canvas, viewport});
-    ctx.scale(dpr, dpr);
-    await page.render({canvasContext: ctx, viewport}).promise;
-
-    // Text layer disabled — re-enable once overlay rendering is resolved
   }
 
   pdfTotalPages = pdf.numPages;
+
+  // Scroll to pivot page immediately — DOM and canvas sizes are set
+  const pivot = Math.max(1, Math.min(pdf.numPages, pivotPage || 1));
+  const pivotEl = document.getElementById(`pdf-page-${pivot}`);
+  if (pivotEl && pivot > 1) pivotEl.scrollIntoView({block: 'start'});
+
+  // Pass 2: render pixels from pivot outward (pivot→end, then pivot-1→start)
+  const order = [];
+  for (let i = pivot - 1; i < pdf.numPages; i++) order.push(i);
+  for (let i = pivot - 2; i >= 0; i--) order.push(i);
+
+  for (const idx of order) {
+    if (_pdfRenderGen !== myGen) { contentEl.innerHTML = ''; return null; }
+    const {page, canvas, viewport} = pageEntries[idx];
+    const ctx = canvas.getContext('2d');
+    ctx.scale(dpr, dpr);
+    await page.render({canvasContext: ctx, viewport}).promise;
+  }
+
   return {title: pdfTitle, author: pdfAuthor, toc, numPages: pdf.numPages};
 }
 
@@ -3698,10 +3711,6 @@ function toggleSettingsPanel() {
 }
 
 async function applyPdfZoom(newZoom) {
-  const contentEl2 = $('reader-content');
-  const fraction = contentEl2
-    ? contentEl2.scrollTop / (contentEl2.scrollHeight - contentEl2.clientHeight || 1)
-    : 0;
   readerSettings.pdfZoom = newZoom;
   const zoomRange = document.getElementById('rs-zoom');
   const zoomVal = document.getElementById('rs-zoom-val');
@@ -3712,25 +3721,32 @@ async function applyPdfZoom(newZoom) {
     pdfSmartZoomPage(pdfCurrentPage);
   } else {
     await reRenderPdf();
-    // Wait for two animation frames so the browser finishes layout
-    // before reading scrollHeight for position restoration
-    await new Promise(r => requestAnimationFrame(() => requestAnimationFrame(r)));
-    if (contentEl2 && fraction > 0) {
-      contentEl2.scrollTop = fraction * (contentEl2.scrollHeight - contentEl2.clientHeight);
   }
 }
+
+function _currentScrollPage(contentEl) {
+  const wrappers = contentEl.querySelectorAll('.pdf-page-wrapper');
+  const viewTop = contentEl.scrollTop;
+  for (const w of wrappers) {
+    if (w.offsetTop + w.offsetHeight > viewTop) {
+      const n = parseInt(w.id.replace('pdf-page-', ''), 10);
+      if (n) return n;
+    }
+  }
+  return pdfCurrentPage || 1;
 }
 
 async function reRenderPdf() {
   if (!currentPdfBuffer) return;
   const contentEl = $('reader-content');
   if (!contentEl) return;
+  const pivot = readerSettings.pdfPaginated ? pdfCurrentPage : _currentScrollPage(contentEl);
   const overlay = $('reader-overlay');
   const loadingEl = document.createElement('div');
   loadingEl.className = 'pdf-loading-overlay';
   loadingEl.innerHTML = '<span class="pdf-loading-spinner"></span>';
   if (overlay) overlay.appendChild(loadingEl);
-  await renderPdf(currentPdfBuffer, contentEl);
+  await renderPdf(currentPdfBuffer, contentEl, undefined, pivot);
   loadingEl.remove();
   if (readerSettings.pdfPaginated) enterPdfPaginatedMode();
 }