how to correctly make flex-box children to scroll - css

Question

I'm stuck and can't make content to correctly scroll while facing a few of nested flex-boxes. Here is my css:
html,body,#root{
width:100%;
height:100%;
padding:0;
margin:0;
}
.flex-fill{
height:100%;
width:100%;
display:flex;
align-items:stretch;
}
.flex-fill>div:last-child{
flex-grow:1;
}
.flex-col{
flex-direction:column;
}
<div id='root'>
<div class='flex-fill flex-col'><!--actually scrolling div-->
<div style="flex:0 0 35px"><h1>main title</h1></div>
<div>
<div class='flex-fill flex-col'>
<div style="flex:0 0 30px"><h2>subtitle</h2></div>
<div class='flex-fill'>
<div style="flex:0 0 30%">...left side nav list...</div>
<div class='flex-fill flex-col'>
<div style="flex:0 0 25px">...data header...</div>
<!--I hope the content of this div is scroll-able-->
<div style="overflow-y:scroll">...data list...</div>
</div>
</div>
</div>
</div>
</div>
</div>
The data list contained in the inner most div is very long and I want to make it scrollable. But in the fact the above codes make whole content of the root div scrolling. Have I misunderstanding about flex or overflow? How can I fix the other parts and just scroll the data list?

Answer 1

Answer updated to keep your HTML structure the same. Inline styles removed and added as classes:
html,body{
height:100%;
padding:0;
margin:0;
}
#root {
height: 100%;
display: flex;
}
.flex-fill{
height:100%;
display:flex;
align-items:stretch;
}
.flex-col{
flex-direction:column;
display: flex;
flex: 1 0 0;
height: 100%;
}
.flex-row{
flex-direction:row;
display: flex;
height: 100%;
}
.scrolling-div {
height: 100%;
overflow-y: scroll;
}
<div id='root'>
<div class='flex-fill flex-col'>
<div><h1>main title</h1></div>
<div class="flex-col">
<div class='flex-col'>
<div><h2>subtitle</h2></div>
<div class="flex-row">
<div>...left side nav list...</div>
<div class='flex-col'>
<div>...data header...</div>
<!--Now Scrolls-->
<div class="scrolling-div">
...scrolling data list...<br />
...scrolling data list...<br />
...scrolling data list...<br />
...scrolling data list...<br />
</div>
</div>
</div>
</div>
</div>
</div>
</div>

Answer 2

#Welson Zhong, please have a look at the below working snippet, please look it in Full page view, hope it helps now :)
note: too much flex kills flex ;) that was the issue with your code.
html, body {
height: 100%;
margin: 0;
padding: 0;
}
body {
display: flex;
flex-direction: column;
font-family: sans-serif;
font-size: 18px;
}
.main {
display: flex;
flex-direction: row;
flex: 1 0 auto;
}
.sidebar {
flex: 1 auto;
}
.content {
display: flex;
flex-flow: column nowrap;
flex: 3 0px;
}
h1,h2,.data-header {
flex: none;
}
/* below css is for visual purpose only */
.main *:not(.content) {
box-shadow: inset 0 0 0 1px #ccc;
padding: 10px;
}
<h1>main title</h1>
<h2>sub title</h2>
<div class="main">
<div class="sidebar">
...left side nav list...
</div>
<div class="content">
<div class="data-header">...data header...</div>
<div style="overflow-y:scroll; flex: auto">...data list...</div>
</div>
</div>