图像已修复，但在div后面消失了

Question

我可以在这里看到我正在寻找的效果，其中fixed_header_middle消失在fixed_header_bottom后面。

＆＃13;

＆＃13;

$(document).ready(function() {
  $(window).bind("scroll", function(e) {
    if (!$('#fixed_header_bottom').hasClass('fixed')) {
      if ($(document).scrollTop() >= 100) {
        $('#fixed_placeholder').show();
        $('#fixed_header_bottom').addClass('fixed');
      }
    } else {
      if ($(document).scrollTop() < 100) {
        $('#fixed_placeholder').hide();
        $('#fixed_header_bottom').removeClass('fixed');
      }
    }
  });
});

＆＃13;

* {
  margin: 0;
  padding: 0;
}
#fixed_header_top {
  display: block;
  position: fixed;
  width: 100%;
  height: 50px;
  background-color: #DD33DD;
  top: 0;
}
#fixed_header_middle {
  display: block;
  width: 100%;
  height: 100px;
  background-color: #DDDD00;
  top: 50px;
  position: fixed;
  z-index: -1;
}
#fixed_header_bottom,
#fixed_placeholder {
  display: block;
  width: 100%;
  height: 50px;
  background-color: #11DD55;
}
#fixed_header_bottom {
  margin-top: 150px;
}
#fixed_placeholder {
  display: none;
}
.fixed {
  position: fixed;
  top: 50px;
  margin-top: 0!important;
}
#body_block {
  background: #FFF;
}

＆＃13;

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="fixed_header_top">fixed_header_top</div>
<div id="fixed_header_middle">fixed_header_middle</div>
<div id="fixed_header_bottom">fixed_header_bottom</div>
<div id="fixed_placeholder">Shouldn't see me</div>
<div id="body_block">BEGIN
  <br />
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />
  <br />END</div>

＆＃13;

＆＃13;

＆＃13;

但是，在此示例中，还有其他效果，例如 fixed_header_bottom变得固定。

我只对在向下滚动时知道如何使固定元素在另一个元素后面消失感兴趣。

Answer 1

如果我得到你想要的东西，z-index就是你要找的...... 它控制元素的叠加.. 见下面的代码

$(document).ready(function() {
  $(window).bind("scroll", function(e) {
    if (!$('#fixed_header_bottom').hasClass('fixed')) {
      if ($(document).scrollTop() >= 100) {
        $('#fixed_placeholder').show();
        $('#fixed_header_bottom').addClass('fixed');
      }
    } else {
      if ($(document).scrollTop() < 100) {
        $('#fixed_placeholder').hide();
        $('#fixed_header_bottom').removeClass('fixed');
$('#fixed_header_bottom').css('z-index', -1);
      }
    }
  });
});

* {
  margin: 0;
  padding: 0;
}
#fixed_header_top {
  display: block;
  position: fixed;
  width: 100%;
  height: 50px;
  background-color: #DD33DD;
  top: 0;
}
#fixed_header_middle {
  display: block;
  width: 100%;
  height: 100px;
  background-color: #DDDD00;
  top: 50px;
  position: fixed;
  z-index: -1;
}
#fixed_header_bottom,
#fixed_placeholder {
  display: block;
  width: 100%;
  height: 50px;
  background-color: #11DD55;
}
#fixed_header_bottom {
  margin-top: 150px;
}
#fixed_placeholder {
  display: none;
}
.fixed {
  position: fixed;
  top: 50px;
  margin-top: 0!important;
}
#body_block {
  background: #FFF;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="fixed_header_top">fixed_header_top</div>
<div id="fixed_header_middle">fixed_header_middle</div>
<div id="fixed_header_bottom">fixed_header_bottom</div>
<div id="fixed_placeholder">Shouldn't see me</div>
<div id="body_block">BEGIN
  <br />
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />Bottom
  <br />
  <br />END</div>