From fce2cfddcf2e07e9f1c50c013789c52494c2a4bd Mon Sep 17 00:00:00 2001
From: Jonas Jenwald <jonas.jenwald@gmail.com>
Date: Sun, 16 Apr 2017 13:10:56 +0200
Subject: [PATCH] Check if `pageView.pdfPage` exists in
 `PDFViewer._ensurePdfPageLoaded`, to avoid potentially calling
 `PDFPageView.setPdfPage` multiple times for the same page

Since calling `PDFPageView.setPdfPage` will in turn call `PDFPageView.reset`, which cancels all rendering and completely resets the page, it's thus possible that we currently cause some unnecessary re-rendering during the initial loading phase of the viewer.

Depending on the order in which data arrives, it's possible (and in practice always seem to happen) that the `pdfPage` property of the *second* page has already been set during `PDFViewer.setDocument`, by the time that the request for the `pdfPage` is resolved in `PDFViewer._ensurePdfPageLoaded`.

Also, note how the `setPdfPage` call in `PDFViewer.setDocument` is already guarded by this kind of check.
---
 web/pdf_viewer.js | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/web/pdf_viewer.js b/web/pdf_viewer.js
index e6ecb2f30..e384ec885 100644
--- a/web/pdf_viewer.js
+++ b/web/pdf_viewer.js
@@ -843,12 +843,13 @@ var PDFViewer = (function pdfViewer() {
       if (this._pagesRequests[pageNumber]) {
         return this._pagesRequests[pageNumber];
       }
-      var promise = this.pdfDocument.getPage(pageNumber).then(
-          function (pdfPage) {
-        pageView.setPdfPage(pdfPage);
+      var promise = this.pdfDocument.getPage(pageNumber).then((pdfPage) => {
+        if (!pageView.pdfPage) {
+          pageView.setPdfPage(pdfPage);
+        }
         this._pagesRequests[pageNumber] = null;
         return pdfPage;
-      }.bind(this));
+      });
       this._pagesRequests[pageNumber] = promise;
       return promise;
     },
@@ -859,9 +860,9 @@ var PDFViewer = (function pdfViewer() {
                                                             this._pages,
                                                             this.scroll.down);
       if (pageView) {
-        this._ensurePdfPageLoaded(pageView).then(function () {
+        this._ensurePdfPageLoaded(pageView).then(() => {
           this.renderingQueue.renderView(pageView);
-        }.bind(this));
+        });
         return true;
       }
       return false;