if (mode == TOUCHPAN) {
      const t = findTouch(e.changedTouches, touchid);
      if (t == null) return;
      if (e.touches.length != 1) {
        setMode(IDLE);
        return;
      }
      panMove(t.clientX, t.clientY);
      e.preventDefault();
    } else if (mode == TOUCHZOOM) {
      // Get two touches; new gap; rescale to ratio.
      const t1 = findTouch(e.touches, touchid);