Mouse scrolling oversize canvas

by vecdeal

HTML

<div id="canvasdiv" style="width: 500px; height: 250px; overflow: hidden">
    <canvas id="canvas" width="10000px" height="250px"></canvas>
</div>
<p>
    Click and drag canvas with mouse
</p>

JavaScript

var canvas = document.getElementById("canvas");
var context = canvas.getContext('2d');
var dragging = false;
var lastX;
var marginLeft = 0;

for (var i = 0; i < 1000; i++) {
    context.beginPath();
    context.arc(Math.random() * 10000, Math.random() * 250, 20.0, 0, 2 * Math.PI, false);
    context.stroke();
} 

canvas.addEventListener('mousedown', function(e) {
    var evt = e || event;
    dragging = true;
    lastX = evt.clientX;
    e.preventDefault();
}, false);

window.addEventListener('mousemove', function(e) {
    var evt = e || event;
    if (dragging) {
        var delta = evt.clientX - lastX;
        lastX = evt.clientX;
        marginLeft += delta;
        canvas.style.marginLeft = marginLeft + "px";
    }
    e.preventDefault();
}, false);

window.addEventListener('mouseup', function() {
    dragging = false;
}, false);