panStart(e.clientX, e.clientY);
    e.preventDefault();
    svg.addEventListener('mousemove', handleScanMove);
  }

  function handleScanMove(e) {
    if (e.buttons == 0) {
      // Missed an end event, perhaps because mouse moved outside window.
      setMode(IDLE);
      svg.removeEventListener('mousemove', handleScanMove);
      return;
    }
    if (mode == MOUSEPAN) panMove(e.clientX, e.clientY);
  }