Update app.css

samples/NodeEditor.Web/AppBundle/app.css

@@ -23,6 +23,17 @@
     pointer-events: none;
 }
 
+canvas {
+    display: block;
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    width: 100%;
+    height: 100%;
+}
+
 #avalonia-splash a{
     color: whitesmoke;
     text-decoration: none;