This is currently not built in. I will add this feature to the list. It shouldn't be difficult to implement it, and there is a clear need for it. So I would expect to see it in future releases, perhaps as soon as SP3.
In the meantime, you will have to write some extra JavaScript code. For example:
function ClearSelectedItem(menu)
{
var curItem = menu.SelectedItem;
if (curItem == null)
{
return;
}
curItem.IsSelected = false;
while (curItem.ParentItem != null)
{
curItem.ParentItem.IsChildSelected = false;
curItem = curItem.ParentItem;
}
menu.SelectedItem = null;
}
function SetSelectedItem(menu,item)
{
item.IsSelected = true;
var curItem = item.ParentItem;
while (curItem != null)
{
curItem.IsChildSelected = true;
curItem = curItem.ParentItem;
}
menu.SelectedItem = item;
}
Please note this is a bit of custom-written code, and we cannot guarantee that it will work with future releases. So if your need is not urgent and/or you are concerned about having to maintain this bit of code, I would wait until a future version when the feature is properly implemented.