Center div (vertically & horizontally) in a 100% height div (Bootstrap) - html

Question

I'm trying to solve my problem since one week, and I really try everything !
I have a two column layout (left: content / right: description of the content).
I want this two columns full height page and the only way I found is :
body {
margin: 0;
height: 100%;
padding: 0;
}
#rightcol {
position: absolute;
top: 0;
bottom: 0;
right: 0;
overflow-y: scroll;
height: 100%;
text-align: right;
}
The closest way to center a div in my columns was to use (in CSS3) flexbox. But there is conflicts with the absolute position of my columns.
Here's the bootply I made to be more explicit :
http://www.bootply.com/1OovYNhx1E#
In this example, I'd like to center (horizontally and vertically) the <h1>TEXT</h1>

Answer 1

UPDATE
Bootply is a terrible tool. So I used Plunker to replicate your example. This includes Bootstrap and everything you had originally except:
.fluid-container and .row are combined.
#inner is now moved out of #leftcol
#inner has the rulesets previously mentioned.
Both columns changed height: 100vh
Added position: relative to body.
Added width:100% and height:100% to html and body elements.
#inner {
position: absolute;
left: 50%;
top: 50%;
bottom: -50%; /* This was added to offset the top: 50% which was keeping the #inner from scrolling any further to the top. */
transform: translate(-50%, -50%);
z-index: 9999;
}
PLUNKER
OLD
Use the following ruleset on your center element:
.center {
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%, -50%);
z-index: 9999;
}
You weren't clear as to where this centered div should be center in relation to. If you want it to be centered in relation to viewport, (i.e. edge to edge of screen) then the center div shouldn't be inside any column. I f you want it centered within the left column, then it's in the correct place. Regardless, if you use this solution it will center itself perfectly inside of whatever you put it into.
SNIPPET
body {
position: relative;
margin: 0;
height: 100%;
padding: 0;
}
#leftcol {
position: absolute;
top: 0;
bottom: 0;
left: 0;
overflow-y: scroll;
height: 100%;
width: 50%;
text-align: left;
background: brown;
}
#rightcol {
position: absolute;
top: 0;
bottom: 0;
right: 0;
overflow-y: scroll;
height: 100%;
width: 50%;
text-align: right;
background: yellow;
}
.center {
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%, -50%);
z-index: 9999;
outline: 3px solid red;
width: 25%;
height: 25%;
background: rgba(0,0,0,.4);
}
<div id='leftcol'></div>
<div class='center'></div>
<div id='rightcol'></div>

Answer 2

Finally find the answer HERE
With flexbox just add to your inner container :
margin: auto;
It will prevent the top scroll problem !