Move the page switching code into set currentPageNumber in `PDFView…

…er` instead of placing it in the `pagechange` event handler

The reason that this code can be moved is that the `if (this.loading && page === 1)` check, in the `pagechange` event handler in viewer.js, is never satisfied since `this.loading` is not defined in that scope.
This *could* be considered a regression from PR 5295, since prior to that `this.loading` was using the `PDFViewerApplication` scope (or `PDFView` as it were).
However, I don't think that we need to fix that since we've been shipping this code in no less than *three* Firefox releases (uplifted in https://bugzilla.mozilla.org/show_bug.cgi?id=1084158), without breaking the world.

An explanation of why the `pagechange` code works, despite `this.loading === undefined`, is that `set currentPageNumber` (in `PDFViewer`) returns early whenever `this.pdfDocument` isn't set. This check is, for all intents and purposes, functionally equivalent to checking `PDFViewerApplication.loading`.
Hence we can move the page switching code into `PDFViewer`, and also remove `PDFViewerApplication.loading` since it's not used any more.
(The `this.loading` property was added in PR 686, which was before the current viewer even existed.)

*Note:* The changes in this patch should also be beneficial to the viewer `components`, since requiring every implementer to provide their own `pagechange` event handler just to get `PDFViewer.currentPageNumber` to actually work seems like an unnecessary complication.

web/pdf_viewer.js

@@ -137,6 +137,12 @@ var PDFViewer = (function pdfViewer() {
       this._currentPageNumber = val;
       event.pageNumber = val;
       this.container.dispatchEvent(event);
+
+      // Check if the caller is `PDFViewer_update`, to avoid breaking scrolling.
+      if (this.updateInProgress) {
+        return;
+      }
+      this.scrollPageIntoView(val);
     },
 
     /**
@@ -566,7 +572,7 @@ var PDFViewer = (function pdfViewer() {
       };
     },
 
-    update: function () {
+    update: function PDFViewer_update() {
       var visible = this._getVisiblePages();
       var visiblePages = visible.views;
       if (visiblePages.length === 0) {
@@ -581,7 +587,7 @@ var PDFViewer = (function pdfViewer() {
 
       this.renderingQueue.renderHighestPriority(visible);
 
-      var currentId = this.currentPageNumber;
+      var currentId = this._currentPageNumber;
       var firstPage = visible.first;
 
       for (var i = 0, ii = visiblePages.length, stillFullyVisible = false;

web/viewer.js

@@ -526,7 +526,6 @@ var PDFViewerApplication = {
     }
 
     var self = this;
-    self.loading = true;
     self.downloadComplete = false;
 
     var passwordNeeded = function passwordNeeded(updatePassword, reason) {
@@ -543,7 +542,6 @@ var PDFViewerApplication = {
                       getDocumentProgress).then(
       function getDocumentCallback(pdfDocument) {
         self.load(pdfDocument, scale);
-        self.loading = false;
       },
       function getDocumentError(exception) {
         var message = exception && exception.message;
@@ -571,7 +569,6 @@ var PDFViewerApplication = {
           message: message
         };
         self.error(loadingErrorMessage, moreInfo);
-        self.loading = false;
       }
     );
 
@@ -1822,16 +1819,6 @@ window.addEventListener('pagechange', function pagechange(evt) {
       Stats.add(page, pageView.stats);
     }
   }
-
-  // checking if the this.page was called from the updateViewarea function
-  if (evt.updateInProgress) {
-    return;
-  }
-  // Avoid scrolling the first page during loading
-  if (this.loading && page === 1) {
-    return;
-  }
-  PDFViewerApplication.pdfViewer.scrollPageIntoView(page);
 }, true);
 
 function handleMouseWheel(evt) {