From V2.57, the ComboBox element can hide its decoration. That means you can make it fully transparent after changing its opacity to 0. This feature is suggested by Ulrich at this thread: http://getsatisfaction.com/easynth/topics/new_attributes_of_the_combobox...
This feature allows you to create your own combo box element. You can use other elements or images as the background of combo box, just like what I did in this example.
Please make sure you are running ForeUI V2.57 or above to see the correct effect.


