Ticket #9117: 9117.patch

_source/core/dom/element.js

@@ -504 +504 @@
                         :
                                 function( propertyName )
                                 {
-                                        return this.getWindow().$.getComputedStyle( this.$, '' ).getPropertyValue( propertyName );
+                                        var style = this.getWindow().$.getComputedStyle( this.$, null );
+
+                                        // Firefox may return null if we call the above on a hidden iframe. (#9117)
+                                        return style ? style.getPropertyValue( propertyName ) : '';
                                 },
 
                 /**