You can achieve this by editing the style of the RadComboBox. Just drop the control on the design surface of Expression Blend, right-click it and choose from the menu Edit Template-> Edit a Copy
. Then Blend will generate all the needed resources. Find the ScrollViewers and apply to them the custom style you are targeting. You can find more information about this type of ScrollViewer in this forum discussion
Hope this helps.
the Telerik team
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