DragSelectionBy
void DragSelectionBy(float x, float y, BRegion *updateRegion);
DragSelectionBy(-x, 0.0, &updateRegion);
DragSelectionBy(x, 0.0, &updateRegion);
DragSelectionBy(0.0, -y, &updateRegion);
DragSelectionBy(0.0, y, &updateRegion);
DragSelectionBy(point.x - box->Frame().left - offset.x,