Try using d3.event.sourceEvent.target:
.on('drag', () => {
const target = d3.event.sourceEvent.target;
const m = target.getCTM();
const x = d3.event.x - this.clickOrigin.x;
const y = d3.event.y - this.clickOrigin.y;
this.setClickOrigin(d3.event);
d3.select(target)
Forked Plunker Example