<view class="page">

<view class="canvas">

    <canvas id="cas" style="width: 410px; height: {{canvasH}}px;" canvas-id="firstCanvas"></canvas>

    </view>

    <view class="bt">

        <button class="save" type="primary">保存</button>

    </view>

</view>

按钮在最底部fix的，只要画布高过屏幕，按钮就被挡住了，z-index也不管用呢？