Decouple LevelRender from window.document

src/level/LevelRender.js

@@ -151,6 +151,7 @@ module.exports = Class( 'LevelRender',
     'private _getObjCanvas': function()
     {
         const canvas     = this._ctx.canvas,
+              document   = this._getDocument( canvas ),
               canvas_obj = document.createElement( 'canvas' );
 
         // mimic the dimensions and positions of the original canvas
@@ -167,6 +168,25 @@ module.exports = Class( 'LevelRender',
     },
 
 
+    /**
+     * Get HTMLDocument node from element ELEMENT
+     *
+     * ELEMENT must be on a DOM.  This allows us to always reference the
+     * proper document node without being coupled with the browser's
+     * window.document, which may not be what we're interested in.
+     *
+     * @param  {HTMLElement}  element  reference element
+     *
+     * @return  {HTMLDocument}  document node
+     */
+    'private _getDocument': function( element )
+    {
+        return ( element.parentElement === null )
+            ? element.parentNode
+            : this._getDocument( element.parentElement );
+    },
+
+
     /**
      * Render the provided level
      *