anchor.diff:  
     _canvasOnTouchStart(Brush brush) =>
         canvas.onTouchStart.listen((TouchEvent e) {
  -        if (e.target == canvas) {
  -          e.preventDefault();
  -        }
  +        e.preventDefault();
   
           final touchPos = _getTouchPos(e);
   
  
positive.code: 
    _documentOnTouchEnd(Brush brush) =>
        document.onTouchEnd.listen((TouchEvent e) {
          if (e.target == canvas) {
            e.preventDefault();
          }
  
          _drawEnd(brush);
        });
  
negative.code: 
    _documentOnTouchMove(Brush brush) =>
        document.onTouchMove.listen((TouchEvent e) {
          e.preventDefault();
  
          final touchPos = _getTouchPos(e);
  
  