I added an empty item at the top so the user can set it to a blank entry but it's displaying a very thin line in the dropdown. (see pic)
Behavior is the same in Chrome and IE11.
The current workaround is to use this but not desirable <telerik:RadComboBoxItem Text="(blank)" Value="" />
Tried without Lightweight rendermode - no difference.
This is within LayoutRow/CompositeLayoutColumn/RadPageLayout shouldn't have an impact just let to let you know.