我有这样的布局：
+---------------------------------+
| #container |
|+-------------------------------+|
|| #top ||
|| This changes height ||
|| dynamically ||
|+-------------------------------+|
|+-------------------------------+|
|| #mid ||
|| ||
|| This changes height ||
|| dynamically ||
|+-------------------------------+|
| |
| |
| |
| |
| |
|+-------------------------------+|
|| #bot ||
|| This has fixed height ||
|| ||
|+-------------------------------+|
+---------------------------------+
当top或mid被放大时，它们会从容器中溢出。我怎样才能做到，这样他们就不会越过bot？当他们想要溢出时，内部 div 应该会出现一个滚动条（又名溢出自动）
CSS
#container{
position: absolute;
} //Container's container has position:relative.
#bot{
position: absolute;
bottom: 0px;
}