Create thumbnails only when the tab is not focused

www/whiteboard/inner.js

@@ -377,6 +377,7 @@ define([
         var initThumbnails = function () {
             var oldThumbnailState;
             var privateDat = metadataMgr.getPrivateData();
+            if (!privateDat.thumbnails) { return; }
             var hash = privateDat.availableHashes.editHash ||
                        privateDat.availableHashes.viewHash;
             var href = privateDat.pathname + '#' + hash;
@@ -389,7 +390,7 @@ define([
                 var D = Thumb.getResizedDimensions($canvas[0], 'pad');
                 Thumb.fromCanvas($canvas[0], D, function (err, b64) {
                     oldThumbnailState = content;
-                    SFUI.setPadThumbnail(href, b64);
+                    Thumb.setPadThumbnail(href, b64);
                 });
             };
             window.setInterval(mkThumbnail, Thumb.UPDATE_INTERVAL);