[例]字体改变,文章位置不变

    function setSize(){

        var lastTop=box.scrollTop;
        var lastHeight=box.scrollHeight;

        // console.log(lastTop,lastHeight,size); // 永远都是500

        var a=(boxHeight*see+lastTop)/lastHeight; // 比例

        box.style.fontSize=size+"px";

        var newTop=0;
        var newHeight=box.scrollHeight;

        newTop=a*newHeight - boxHeight*see;

        box.scrollTop=Math.round(newTop);
    };