Canvas无法绘制IE 11

Question

我有一个画布代码来绘制签名。该代码在chrome和Firefox上运行得很好，但在IE11上完全不能绘制。

我的画布是：

<canvas id="signitureCanvas" style="border: 3px solid #000; cursor:crosshair; background-color:white;"></canvas>

我的代码如下：

 var canvas = document.getElementById('signitureCanvas');
    var ctx = canvas.getContext('2d');
    var canvasWidth = 200;
    var canvasLength = 120;
    canvas.width = canvasWidth;
    canvas.height = canvasLength;
    var canvasx = $(canvas).offset().left;
    var canvasy = $(canvas).offset().top;
    var last_mousex = last_mousey = 0;
    var mousex = mousey = 0;
    var mousedown = false;
    var tooltype = 'draw';

    //Mousedown
    $(canvas).on('mousedown', function (e) {
        last_mousex = mousex = parseInt(e.clientX - canvasx);
        last_mousey = mousey = parseInt(e.clientY - canvasy);
        mousedown = true;
    });

    //Mouseup
    $(canvas).on('mouseup', function (e) {
        mousedown = false;
    });

    //Mousemove
    $(canvas).on('mousemove', function (e) {
        mousex = parseInt(e.clientX - canvasx);
        mousey = parseInt(e.clientY - canvasy);
        if (mousedown) {
            ctx.beginPath();
            if (tooltype == 'draw') {
                ctx.globalCompositeOperation = 'source-over';
                ctx.strokeStyle = 'black';
                ctx.lineWidth = 3;
            } else {
                ctx.globalCompositeOperation = 'destination-out';
                ctx.lineWidth = 10;
            }
            ctx.moveTo(last_mousex, last_mousey);
            ctx.lineTo(mousex, mousey);
            ctx.lineJoin = ctx.lineCap = 'round';
            ctx.stroke();
        }
        last_mousex = mousex;
        last_mousey = mousey;
    });

    function ClearCanvas() {
        var canvas = document.getElementById('signitureCanvas');
        var ctx = canvas.getContext('2d');
        ctx.clearRect(0, 0, canvas.width, canvas.height);
    }

IE 11是不是特别有问题？

编辑：

我发现问题出在我的Iframe上：

当高度和宽度设置为300时，一切正常：

 <embed id="fred"  type="application/pdf" style="border:1px solid #666CCC" title="PDF in an i-Frame" src="@Model.FilePath" frameborder="1" scrolling="yes" height="300" width="300" />

当我将其设置为1000时，它将不起作用：

 <embed id="fred"  type="application/pdf" style="border:1px solid #666CCC" title="PDF in an i-Frame" src="@Model.FilePath" frameborder="1" scrolling="yes" height="1000" width="1000" />

我相信是有偏移量的问题，但我想不出该怎么解决。

有什么帮助吗？