To reproduce:
- Add RadPageView change the view  to ExplorerBar and add some pages with controls.
- Start the application expand all pages and scroll.

Workaround:
class MyPageView : RadPageView
{
    protected override RadPageViewElement CreateUI()
    {
        if (this.ViewMode == PageViewMode.ExplorerBar)
        {
            return new MyExplorerBarElement();
        }
        return base.CreateUI();
    }
   
}

class MyExplorerBarElement : RadPageViewExplorerBarElement
{
    protected override bool IsChildElementExternal(Telerik.WinControls.RadElement element)
    {
        return !(element is RadPageViewElementBase) && base.IsChildElementExternal(element);
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadPageViewExplorerBarElement);
        }
    }
}