If the itemssource for a combobox is set to a large number of items sometimes when you expand the dropdown it has no border. Then if you collapse it and re-expand it the border will appear.

Attached are two images of the same dropdown, one with the border and one without.

You can continually open and close the dropdown to see that sometimes the border is there and sometimes it is not.

Thanks in advance for your help!

Hi Eric,

I tried to reproduce the issue but with no success. Could you please provide some more information about your  scenario. How much items do you have, do you use Virtualizing Stackpanel or any custom styles/templates?

If it is possible you can send a simple application where the issue can be reproduced. It will be of great help in pinpointing the problem.

Greetings,
Boyan
the Telerik team