When you have set SelectionMode="Extended", then selection is corrupted if RadGridView looses focus during extended selection.
That is how RadGridView handles the Drag selection. It keeps selecting until the Mouse Button is released. Unfortunately there is no way to know if the Mouse button has been released over the displayed Popup.
Solution 1: Set RadGridView's property DragElementAction to None.
Solution 2: You can disable the user selection right after you have entered the event handler. After your logic is executed you have to re-enable the user selection. This can be controlled through setting RadGridView.CanUserSelect property.