This item is declined for a couple of reasons, explaining why the editor is closed when a user clicks on the scroll bar:
1. Because of the RadGridView UI virtualization if you start scrolling with an editor open the cell containing the editor will be cached and then reused for another data cell. As a result, the editor will jump all over the visual cells of RadGridView.
2. If the editor remains open and you scroll it out of view, validation will be hard to convey to the users. If the edited cell is out of view and validation fails there is no clear way to indicate that to the user.