public class CustomPropertyGridEditor : RadPropertyGrid
{
    protected override PropertyGridElement CreatePropertyGridElement()
    {
        return new CustomPropertyGridElement();
    }
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadPropertyGrid).FullName;
        }
    }
}
public class CustomPropertyGridElement : PropertyGridElement
{
    protected override PropertyGridSplitElement CreateSplitElement()
    {
        return new CustomPropertyGridSplitElement();
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(PropertyGridElement);
        }
    }
}
public class CustomPropertyGridSplitElement : PropertyGridSplitElement
{
    protected override PropertyGridTableElement CreateTableElement()
    {
        return new CustomPropertyGridTableElement();
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(PropertyGridSplitElement);
        }
    }
}
public class CustomPropertyGridTableElement : PropertyGridTableElement
{
    public override bool ProcessMouseUp(MouseEventArgs e)
    {
        bool selectionWasCanceled = false;
        var field = typeof(PropertyGridTableElement).GetField("selectionWasCanceled", 
            System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
        if (field != null)
        {
            selectionWasCanceled = (bool)field.GetValue(this);
        }

        if (selectionWasCanceled)
        {
            (this.Parent as PropertyGridSplitElement).CallDoMouseUp(e);
            return true;
        }
        return base.ProcessMouseUp(e);
    }

    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(PropertyGridTableElement);
        }
    }
}