Starting with the node indicated by the Selected property, FindNextToSelect searches for a deselected node, with preference given to subsequent siblings. If the Selected node and all its siblings are selected, the search proceeds to the parent node and its siblings, and so on up the tree. If the top level is reached without finding a deselected node, nil is returned. 

Note that the Selected property can refer to a deselected node when MultiSelect is true. In this case, FindNextToSelect returns that node. 

Selected 

MultiSelect