To reproduce: this issue is reproducible under Windows8 OS only
-add RadTreeView from the Toolbox to the Form and add several tree nodes at design time;
-add RadContextMenu from the Toolbox and add several items at design time;
Use the following code:
public Form1()
{
    InitializeComponent();

    this.radTreeView1.RadContextMenu = this.radContextMenu1;
}
-run the application;
-select a tree node;
-right mouse click over the node to show the context menu; As a result the main form looses focus and you are unable to click anywhere outside the RadTreeView.
This issue is inconstant and it is not appearing every time.

Workaround: use ContextMenu property instead:
public Form1()
{
    InitializeComponent();

    ContextMenu menu = new ContextMenu();

    foreach (var item in radContextMenu1.Items)
    {
        menu.MenuItems.Add(new MenuItem(item.Text));
    }

    this.radTreeView1.ContextMenu = menu;
    // this.radTreeView1.RadContextMenu = this.radContextMenu1;
}