Update thumbnail style only when sidebar is visible

web/pdf_thumbnail_viewer.js

@@ -76,7 +76,9 @@ var PDFThumbnailViewer = (function PDFThumbnailViewerClosure() {
         selected.classList.remove('selected');
       }
       var thumbnail = document.getElementById('thumbnailContainer' + page);
-      thumbnail.classList.add('selected');
+      if (thumbnail) {
+        thumbnail.classList.add('selected');
+      }
       var visibleThumbs = this._getVisibleThumbs();
       var numVisibleThumbs = visibleThumbs.views.length;
 

web/viewer.js

@@ -1574,6 +1574,10 @@ function webViewerInitialized() {
       outerContainer.classList.toggle('sidebarOpen');
       PDFViewerApplication.sidebarOpen =
         outerContainer.classList.contains('sidebarOpen');
+      if (PDFViewerApplication.sidebarOpen) {
+        PDFViewerApplication.pdfThumbnailViewer.
+          scrollThumbnailIntoView(PDFViewerApplication.page);
+      }
       PDFViewerApplication.forceRendering();
     });
 
@@ -1920,7 +1924,9 @@ window.addEventListener('pagechange', function pagechange(evt) {
   var page = evt.pageNumber;
   if (evt.previousPageNumber !== page) {
     document.getElementById('pageNumber').value = page;
-    PDFViewerApplication.pdfThumbnailViewer.scrollThumbnailIntoView(page);
+    if (PDFViewerApplication.sidebarOpen) {
+      PDFViewerApplication.pdfThumbnailViewer.scrollThumbnailIntoView(page);
+    }
   }
   var numPages = PDFViewerApplication.pagesCount;