I'm answering my own question. You can click-and-drag to select a range of chords but you must initiate the click in a blank space in the chord track that is adjacent to the group of chords you intend to move. So, if you have a continuous sequence of chords and need to move only some of them, you must first click and drag a chord at one end of the group to be moved and shrink it horizontally thus creating some blank space. Then you can click-and-drag from the blank space across the group of chords to be moved.