Fabric JS - Issue in Scrollable Div

Fabric JS - Issue in Scrollable Div

HTML

<script src="https://rawgit.com/kangax/fabric.js/master/dist/fabric.js"></script>
<canvas id="a" width="400" height="400"></canvas>

CSS

canvas {
    border: 1px solid #999;
}

JavaScript

/**
 * fabric.js template for bug reports
 *
 * Please update the name of the jsfiddle (see Fiddle Options).
 * This templates uses latest dev verison of fabric.js (https://rawgithub.com/kangax/fabric.js/master/dist/all.js).
 */

// initialize fabric canvas and assign to global windows object for debug
var canvas = window._canvas = new fabric.Canvas('a', {
			backgroundColor: '#FFF',
		});
		
		canvas.canvas_h = 400;
		canvas.canvas_w = 400;
		canvas.freeDrawingBrush.width = 2;
		canvas.freeDrawingBrush.color = '#005E7A';
		canvas.setHeight(canvas.canvas_h).setWidth(canvas.canvas_w);
        canvas.controlsAboveOverlay = true;


// add red rectangle
var rect1 = new fabric.Rect({
  width: 80,
  height: 50,
  left: 60,
  top: 5,
  fill: 'black',
  stroke: 'black'
});

// add green, half-transparent circle
var circle = new fabric.Circle({
  radius: 60,
  left: 260,
  top: 5,
  fill: null,
  stroke: 'black'
});

var circle2 = new fabric.Circle({
  radius: 60,
  left: 100,
  top: 200,
  fill: 'black',
  stroke: 'black'
});

fabric.Image.fromURL('http://fabricjs.com/assets/pug.jpg', function(img) {
    canvas.add(img);
    canvas.add(circle);
});
    canvas.clipTo = function(ctx) {
        circle2.render(ctx);
        circle.render(ctx);
    }

//canvas.add(rect1);