try to fix iframe border issue

build/templates/example.template

@@ -1,5 +1,5 @@
 <div class="threejs_example_container">
-  <iframe class="threejs_example" style="{{width}} {{height}}" src="/threejs/resources/editor.html?url={{{examplePath}}}{{{encodedUrl}}}{{{params}}}"></iframe>
+  <div><iframe class="threejs_example" style="{{width}} {{height}}" src="/threejs/resources/editor.html?url={{{examplePath}}}{{{encodedUrl}}}{{{params}}}"></iframe></div>
   <a class="threejs_center" href="{{{examplePath}}}{{{url}}}" target="_blank">{{{caption}}}</a>
 </div>
 

threejs/lessons/resources/lesson.css

@@ -305,11 +305,14 @@ iframe.threejs_example, iframe.threejs_diagram {
   display: block;
 }
 iframe.threejs_example {
-  border: 1px solid black;
+  background: black;
 }
 iframe.noborder {
   border: none !important;
 }
+.threejs_example_container>div {
+  border: 1px solid black;
+}
 
 iframe.external_diagram {
   border: none;