主要是需要留意 flex布局和overflow 之间的关系, 
  最外面的container 和 里面的main-box 之间分别使用了flex布局 和 overflow:hidden main-box 和 里面的main-body 之间分别使用了 flex布局 和 overflow:auto    有点类似于聊天的窗口布局   
<! DOCTYPE  html >  
< html  lang = " en" >  
< head>  
    < meta  charset = " UTF-8" >  
    < meta  http-equiv = " X-UA-Compatible"   content = " IE=edge" >  
    < meta  name = " viewport"   content = " width=device-width, initial-scale=1.0" >  
    < title>  Document</ title>  
    < style> 
        body  { 
            margin :  0; 
        } 
        .container   { 
            width :  800px; 
            height :  600px; 
            margin :  auto; 
            background-color :  #bfa; 
            display :  flex; 
            flex-direction :  column; 
        } 
        .container-title  { 
            border :  3px solid red; 
            height :  40px; 
        } 
        .main-box  { 
            flex :  1; 
            overflow :  hidden; 
            border :  3px solid blue; 
            display :  flex; 
            flex-direction :  column; 
        } 
        
        .main-header  { 
            height :  80px; 
            border :  3px solid pink; 
            flex-shrink :  0; 
        } 
        .main-body  { 
            border :  3px solid pink; 
            flex :  1 1 auto; 
            overflow-y :  auto; 
        } 
        .main-body-long-content  { 
            height :  600px; 
        } 
        .main-footer  { 
            height :  80px; 
            border :  3px solid pink; 
            flex-shrink :  0; 
        } 
        
     </ style>  
</ head>  
< body>  
	
    < div  class = " container" >  
    
    	
        < div  class = " container-title" >  容器标题 - 高40px</ div>  
        
        
        < div  class = " main-box" >  
        
        	
            < div  class = " main-header" >  
                我是标题 - 80px
            </ div>  
            
            
            < div  class = " main-body" >  
                < div  class = " main-body-long-content" >  
                    很的长内容...
                    (这里可以用el-scrollbar)
                </ div>  
            </ div>  
            
            
            < div  class = " main-footer" >  
                我是尾部 - 80px
            </ div>  
            
        </ div>  
        
    </ div>  
    
</ body>  
</ html>