css position属性与js滚动

“视口”就是浏览器窗口中实际显示文档内容的区域，不包含浏览器的“外框”，如菜单、工具条和标签。文档则是指整个网页。

1 css 的position

static	正常定位，是元素position属性的默认值，元素遵循常规流。
relative	相对定位，会相对于自身在常规流中的位置进行定位。其在常规流中的位置会被保留。
absolute	绝对定位，会脱离文档流，相对于离自身最近的祖先（position为relative）进行偏移定位。
fixed	固定定位，会脱离文档流，相对窗口进行偏移定位。
sticky	粘性定位，类似static和fixed的结合。

表 css position属性的5个值

1.1 relative 相对定位

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title>
</head>
<body>
<div class="container"><div class="con1"><div class="block"></div><div class="block">position:static</div><div class="block"></div></div><div class="isolate"></div><div class="con2"><div class="block block1"></div><div class="block block2">position:relative</div><div class="block"></div></div>
</div>
</body>
</html><style lang="less">.block {width: 150px;height: 100px;background-color: #f3bbbb;border: solid 1px blue;text-align: center;line-height: 100px;color: red;}.container {display: flex;.isolate{width: 30px;}.con2 {.block1 {z-index: 99;}.block2 {z-index: 1;background-color: blue;position: relative;top: -100px;}}}
</style>

图 position=relative 示意图

注意，当position为static(常规)时，z-index及left(等位置元素)均为无效。

1.2 absolute 绝对定位

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title>
</head>
<body>
<div class="container"><div class="con1"><div class="block"></div><div class="block">position:static</div><div class="block"></div></div><div class="isolate"></div><div class="con2"><div class="block block1"></div><div class="block block2"><div>position:absolute (parent is "body")</div></div><div class="block"></div></div><div class="isolate"></div><div class="con2"><div class="block block1"></div><div class="parent"><div class="block block2">position:absolute (parent is "div")</div></div><div class="block"></div></div>
</div>
</body>
</html><style lang="less">.block {width: 150px;height: 100px;background-color: #f3bbbb;border: solid 1px blue;text-align: center;line-height: 100px;color: red;}.container {display: flex;.isolate{width: 30px;}.con2 {.block1 {z-index: 99;}.block2 {line-height: 50px;z-index: 1;background-color: blue;position: absolute;top: 50px;}.parent {position: relative;}}}
</style>

图 position=absolute 示意图

注意: absolute 是相对于position=relative的最近父元素定位，当没有找到相关父元素时，会相对于<body>标签定位。

1.3 fixed 固定定位

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title>
</head>
<body>
<div class="block"><div class="con"></div>
</div>
</body>
</html><style>
.block {width: 10000px;height: 600px;background: #f3c4c4;
}
.con {width: 100px;height: 100px;position: fixed;background: blue;left: 100px;top: 100px;
}
</style>

图 position=fixed示意图

注意：fixed 是相对于窗口，会脱离文档流。

1.4 sticky 粘性定位

当position=sticky时，如果top、right、bottom、left四个属性都不设置具体值，sticky不会生效，其表现效果与static一致。

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>Title</title>
</head>
<body>
<div class="big-container"><div class="container container1"><div class="con con1"></div><div class="sep"></div><div class="con con2"></div><div class="sep"></div><div class="con con3"></div></div><div class="container container2"><div class="con con1"></div><div class="sep"></div><div class="con con2"></div><div class="sep"></div><div class="con con3"></div></div>
</div>
</body>
</html><style>.big-container {display: flex;.container {width: 100px;height: 250px;margin-left: 30px;border: solid 1px #8a8a8a;overflow: auto;.sep {width: 100px;height: 30px;background: grey;}&.container2 {.con2 {top: 30px;position: sticky;}}.con {width: 100px;height: 100px;background: #f3bbbb;}.con2 {background: green;}}}
</style>