微信浏览器上拉出现黑块，导致滚动条卡顿问题

function overscroll(el) {

el.addEventListener('touchstart',function() {

var top = el.scrollTop

,totalScroll = el.scrollHeight

,currentScroll = top + el.offsetHeight;

//If we're at the top or the bottom of the containers

//scroll, push up or down one pixel.

// 参数为添加有overflow:scroll的dom元素

//this prevents the scroll from "passing through" to

//the body.

if(top ===0) {

el.scrollTop=1;

}else if(currentScroll === totalScroll) {

el.scrollTop= top -1;

}

});

el.addEventListener('touchmove',function(evt) {

//if the content is actually scrollable, i.e. the content is long enough

//that scrolling can occur

if(el.offsetHeight< el.scrollHeight)

evt._isScroller=true;

});

document.body.addEventListener('touchmove',function(evt) {

//In this case, the default behavior is scrolling the body, which

//would result in an overflow.  Since we don't want that, we preventDefault.

if(!evt._isScroller) {

evt.preventDefault();

}

});

}