Hi Zoltan,
The ComboDropDown uses a PopupControlContainer as its dropdown. To prevent the PopupControlContainer from closing when it loses focus, please refer to the following KB article :
How do I prevent the popup from disappearing when the user clicks on a Control (that is not a "popup parent")?
Let me know if you need any other information.
Regards,
Guru Patwal
Syncfusion, Inc.