Merge pull request #14096 from Snuffleupagus/spreadMode-preRender

Pre-render *one* additional page when spreadModes are enabled

Author: timvandermeij

web/base_viewer.js

@@ -1250,10 +1250,16 @@ class BaseViewer {
     const scrollAhead = this._isScrollModeHorizontal
       ? this.scroll.right
       : this.scroll.down;
+    const preRenderExtra =
+      this._scrollMode === ScrollMode.VERTICAL &&
+      this._spreadMode !== SpreadMode.NONE &&
+      !this.isInPresentationMode;
+
     const pageView = this.renderingQueue.getHighestPriority(
       visiblePages,
       this._pages,
-      scrollAhead
+      scrollAhead,
+      preRenderExtra
     );
     if (pageView) {
       this._ensurePdfPageLoaded(pageView).then(() => {

web/pdf_rendering_queue.js

@@ -102,8 +102,9 @@ class PDFRenderingQueue {
    * @param {Object} visible
    * @param {Array} views
    * @param {boolean} scrolledDown
+   * @param {boolean} [preRenderExtra]
    */
-  getHighestPriority(visible, views, scrolledDown) {
+  getHighestPriority(visible, views, scrolledDown, preRenderExtra = false) {
     /**
      * The state has changed. Figure out which page has the highest priority to
      * render next (if any).
@@ -126,20 +127,20 @@ class PDFRenderingQueue {
       }
     }
 
-    // All the visible views have rendered; try to render next/previous pages.
-    if (scrolledDown) {
-      const nextPageIndex = visible.last.id;
-      // IDs start at 1, so no need to add 1.
-      if (views[nextPageIndex] && !this.isViewFinished(views[nextPageIndex])) {
-        return views[nextPageIndex];
-      }
-    } else {
-      const previousPageIndex = visible.first.id - 2;
-      if (
-        views[previousPageIndex] &&
-        !this.isViewFinished(views[previousPageIndex])
-      ) {
-        return views[previousPageIndex];
+    // All the visible views have rendered; try to render next/previous page.
+    // (IDs start at 1, so no need to add 1 when `scrolledDown === true`.)
+    let preRenderIndex = scrolledDown ? visible.last.id : visible.first.id - 2;
+    let preRenderView = views[preRenderIndex];
+
+    if (preRenderView && !this.isViewFinished(preRenderView)) {
+      return preRenderView;
+    }
+    if (preRenderExtra) {
+      preRenderIndex += scrolledDown ? 1 : -1;
+      preRenderView = views[preRenderIndex];
+
+      if (preRenderView && !this.isViewFinished(preRenderView)) {
+        return preRenderView;
       }
     }
     // Everything that needs to be rendered has been.