//////////求一个节点在有滚动条包裹的父节点里面，offset().top的值







p在div同级



p在div里面包裹着 ，会所父节点div滚动，影响offset的值，应该要加上滚动的值


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试着移动滚动条，然后再次点击按钮。




/////////////////求window的scrollTop()的值

如果没有滚动条的时候，$(window).scrollTop()是0，这时候的window的高度通过$(window).height()取得；

如果有滚动条出现，滚动的搞定要加上window的高度

$(window).scrollTop()+$(window).height()，这才是滚动了多少高度