I have tested the provided ComboBox Demo and I have found the cause of the issue. In fact, this is not related to the ComboBox widget, but rather to the CSS layout of the page and more specifically to the <label> which is above the widget. Here is a short screencast
, which demonstrates this behavior.
Can you please verify that this is not the case? If the issue continues to persist, you can open a separate support thread and provide a sample solution, where the described issue is reproduced, so that we can have a look and advise you further.