To reproduce
- Click between the thumb and the button.

Workaround:
public class MyDiagramInputBehavior : DiagramInputBehavior
{
    public MyDiagramInputBehavior(RadDiagramElement element) : base(element)
    {


    }
    protected override bool IsScrollBar(RadElement element)
    {
        if (element != null && element is RadScrollBarElement)
        {
            return true;
        }
        return base.IsScrollBar(element);
    }
}