I recently upgraded my application from the 2016 controls to the 2017 R2 controls and now I'm having issues with the Combobox and how it displays on the screen.
My combobox is within an Editform / form template of a RadGrid. It is defined this way:
<telerik:RadComboBox ID="rddRouteInner" runat="server" DataTextField="route" DataValueField="route"
DropDownHeight="200px" DropDownWidth="350px" AllowCustomText="true" Font-Size="12px" >
Very simple - no CSS applied or anything else. I added the Font-Size="12 px" because the font in the input was different from the drop down since the upgrade - which seems kind of crazy - but not a big deal.
Attached is what the output looks like. The Input Box is taller than the surrounding drop down box - and I get multiple drop down boxes (at least started). I have no idea how to troubleshoot this - when I just put a combobox on a page by itself - it works correctly.