Fix intermittent blank page on fast zoom.

src/api.js

@@ -852,7 +852,7 @@ var InternalRenderTask = (function InternalRenderTaskClosure() {
     cancel: function InternalRenderTask_cancel() {
       this.running = false;
       this.cancelled = true;
-      this.callback();
+      this.callback('cancelled');
     },
 
     operatorListChanged: function InternalRenderTask_operatorListChanged() {

web/viewer.js

@@ -1560,7 +1560,6 @@ var PageView = function pageView(container, id, scale,
   this.update = function pageViewUpdate(scale, rotation) {
     if (this.renderTask) {
       this.renderTask.cancel();
-      this.renderTask = null;
     }
     this.resume = null;
     this.renderingState = RenderingStates.INITIAL;
@@ -1828,8 +1827,18 @@ var PageView = function pageView(container, id, scale,
 
     var self = this;
     function pageViewDrawCallback(error) {
-      self.renderingState = RenderingStates.FINISHED;
+      // The renderTask may have been replaced by a new one, so only remove the
+      // reference to the renderTask if it matches the one that is triggering
+      // this callback.
+      if (renderTask === self.renderTask) {
         self.renderTask = null;
+      }
+
+      if (error === 'cancelled') {
+        return;
+      }
+
+      self.renderingState = RenderingStates.FINISHED;
 
       if (self.loadingIconDiv) {
         div.removeChild(self.loadingIconDiv);
@@ -1890,7 +1899,7 @@ var PageView = function pageView(container, id, scale,
         cont();
       }
     };
-    this.renderTask = this.pdfPage.render(renderContext);
+    var renderTask = this.renderTask = this.pdfPage.render(renderContext);
 
     this.renderTask.then(
       function pdfPageRenderCallback() {