We can mass-select pieces within a bounding box by clicking and dragging. We can add / remove single pieces from the selection by ctrl+click.
Suggestion: Add a way to deselect pieces with a bounding box, if it doesn't already exist. I tried Ctrl+clickdrag (adds to existing selection, but doesn't remove from it), Ctrl+Shift+clickdrag and Ctrl+Alt+clickdrag.
Use case: A line of thin terrain pieces need to be selected. These overlap a considerable number of other pieces, so would need a click-drag to select the thin pieces, then some means to unselect the other pieces. Alternative is selecting (or deselecting) each individual piece by hand.