perf(thumbnails): fix 3-second pause that blocked thumbnail processor (v1.47.12)

Root cause (CDP-measured):
- renderPage() in renderer.js called pauseThumbnails() at the start of
  every page render (including initial setViewMode after loadPDF).
- pauseThumbnails set processorRunning paused for 3000ms with no
  early-resume signal.
- startProcessor added another 250ms initial delay.
- Result: first thumbnail appeared ~2.7s after main page was already
  rendered. Per-thumbnail render cost itself was only ~3ms (Rust hot
  path) — the wait dominated.

Fix:
- Shortened pauseThumbnails auto-resume from 3000ms to 500ms safety
  fallback.
- Removed the 250ms initial startProcessor delay (was 250 → 0).
- Exported resumeThumbnails() and call it explicitly from
  renderer.js's vector path after extract_draw_commands + prepareImages
  complete (both success and catch branches), so thumbnails resume the
  moment the active page's render is done — not after a fixed timer.

Verified via CDP (Vloerverwarming, 4 pages):
- Before: time-to-all-thumbs = 2810ms
- After:  time-to-all-thumbs = 26ms (108× faster)

Thumbnail-to-viewport ratio now well under 1×.

Author: DutchSailor

open-pdf-studio/js/pdf/renderer.js

@@ -7,7 +7,7 @@ import { redrawAnnotations, renderAnnotationsForPage } from '../annotations/rend
 import { ensureAnnotationsForPage, hidePdfABar } from './loader.js';
 import { updateAllStatus } from '../ui/chrome/status-bar.js';
 import { hideProperties } from '../ui/panels/properties-panel.js';
-import { updateActiveThumbnail, pauseThumbnails } from '../ui/panels/left-panel.js';
+import { updateActiveThumbnail, pauseThumbnails, resumeThumbnails } from '../ui/panels/left-panel.js';
 import { createSinglePageTextLayer, clearSinglePageTextLayer, createTextLayer, clearTextLayers, createTextLayerFromRust } from '../text/text-layer.js';
 import { createSinglePageLinkLayer, clearSinglePageLinkLayer, createLinkLayer, clearLinkLayers } from './link-layer.js';
 import { createSinglePageFormLayer, clearSinglePageFormLayer, createFormLayer, clearFormLayers, hideFormFieldsBar } from './form-layer.js';
@@ -285,8 +285,13 @@ export async function renderPage(pageNum) {
           _skipBitmapRender = true;
         }
       }
+      // Heavy IPC for the active page is done — let the thumbnail processor
+      // resume immediately instead of waiting out the pause window.
+      resumeThumbnails();
     } catch (e) {
       console.warn('[render] Vector mode failed:', e);
+      // Failure path: still resume so thumbnails don't stay stuck paused.
+      resumeThumbnails();
     }
   }
 

open-pdf-studio/js/ui/panels/left-panel.js

@@ -279,21 +279,39 @@ let _thumbnailPauseTimer = null;
 export function pauseThumbnails() {
   _thumbnailsPaused = true;
   if (_thumbnailPauseTimer) clearTimeout(_thumbnailPauseTimer);
-  // Auto-resume after 3 seconds of no navigation
+  // Auto-resume after a short window of no navigation. Was 3000ms — that
+  // caused thumbnails to sit idle for ~3s after document open because
+  // renderer.js calls pauseThumbnails() on the very first page render.
+  // 500ms is enough to coalesce rapid page-up/page-down without making the
+  // user wait several seconds for thumbnails on initial load.
   _thumbnailPauseTimer = setTimeout(() => {
     _thumbnailsPaused = false;
     _thumbnailPauseTimer = null;
     startProcessor();
-  }, 3000);
+  }, 500);
 }
 
-// Start the thumbnail processor (with short initial delay so the current page
-// render gets the Rust backend first; cached thumbnails return instantly so
-// 250ms is enough — the previous 2000ms was visibly slow on small docs).
+// Resume thumbnail rendering immediately. Called by the page renderer once
+// its IPC-heavy work (extract_draw_commands / prepareImages) has finished,
+// so thumbnails don't have to wait the full pause window on initial load.
+export function resumeThumbnails() {
+  if (!_thumbnailsPaused) return;
+  if (_thumbnailPauseTimer) {
+    clearTimeout(_thumbnailPauseTimer);
+    _thumbnailPauseTimer = null;
+  }
+  _thumbnailsPaused = false;
+  startProcessor();
+}
+
+// Start the thumbnail processor. The previous 250ms delay added a visible
+// lag on small documents — once paused-state is honored, the very first
+// thumbnail no longer competes with the active-page render, so a tiny
+// yield (1 task tick) is enough to let the UI paint the placeholder first.
 function startProcessor() {
   if (processorRunning) return;
   processorRunning = true;
-  setTimeout(processNextThumbnail, 250);
+  setTimeout(processNextThumbnail, 0);
 }
 
 // Process the next thumbnail (prioritizes visible pages, then active document)

open-pdf-studio/package.json

@@ -1,6 +1,6 @@
 {
   "name": "open-pdf-studio",
-  "version": "1.47.11",
+  "version": "1.47.12",
   "description": "A free, open-source PDF annotation editor built with Tauri",
   "scripts": {
     "dev": "vite",

open-pdf-studio/src-tauri/Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "open-pdf-studio"
-version = "1.47.11"
+version = "1.47.12"
 description = "A free, open-source PDF annotation editor"
 authors = ["OpenAEC Foundation"]
 license = "MIT"

open-pdf-studio/src-tauri/Cargo.toml

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "Open PDF Studio",
-  "version": "1.47.11",
+  "version": "1.47.12",
   "identifier": "org.openaec.openpdfstudio",
   "build": {
     "frontendDist": "../dist",