Hi guys,

This is caused by a framework behavior - it can be reproduced by using the following code snippet:

<ScrollViewer CanContentScroll="True" Height="100">
    <StackPanel>
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" />
        <TextBlock Text="1" Visibility="Collapsed" />
        <TextBlock Text="1" Visibility="Collapsed" />
        <TextBlock Text="1" Visibility="Collapsed" />
        <TextBlock Text="1" Visibility="Collapsed" />
    </StackPanel>
</ScrollViewer>

As workaround you could set the attached ScrollViewer.CanContentScroll property of RadComboBox to False:

<telerik:RadComboBox ScrollViewer.CanContentScroll="False" />

However we also logged this in MS Connect, so you can vote for the item on the following link:
https://connect.microsoft.com/VisualStudio/feedback/details/2705461

Regards,
Kalin