How to give a fixed position div a z-index - css-position

Question

I'm working on a website that has a lot of z-index values and I'm trying to make a Drag & Drop menu that you can move around but stays in the screen space at all times. But because the drag & drop menu would have a fixed position it breaks the z-index positioning (it reveals borders that it wouldn't if it was positioned absolute).
I understand that you can't position a fixed element with z-index but do you guys maybe know a workaround for it?
Here is the JS fiddle of what I have so far (I left the header in):
https://jsfiddle.net/wdeyvb7q/
HTML:
<div id="menu-container">
<div id="draggable3" class="draggable ui-widget-content">
<p>I'm a very confused box, position fixed on my container breaks the style.</p>
</div>
</div>
CSS (with #menu-container absolute):
#menu-container {
width: calc(90vw - 94px);
height: calc(100vh + 8px);
top: -4px;
position: absolute;
left: calc(5vw + 47px);
}
.draggable {
background: white;
width: 100%;
height: 90px;
float: left;
position: absolute;
z-index: 200;
border-top: 4px solid black;
border-bottom: solid black;
}
#draggable, #draggable2 {
margin-bottom:0px;
}
#draggable {
cursor: n-resize;
}
JS:
$( function() {
$( "#draggable3" ).draggable({ containment: "#menu-container", scroll: false });
} );
And here are 2 screenshots of an absolute & fixed position

Answer 1

I solved it! I removed both inner borders of the sidebars and added 2 divs floating left and right. By positioning those divs under the menu code in the HTML file it will stay under the menu, so I added a border on each side and that didn't break the design =)!
JSFiddle: https://jsfiddle.net/adf2gte7/
HTML:
<!-- Left And Right Inner Borders -->
<div class='left-border-menu'></div>
<div class='right-border-menu'></div>
CSS:
/* Inner Borders */
.left-border-menu {
width: calc(5vw + 47px);
height: 2000px;
background: orange;
float: left;
border-right: 4px solid black;
}
.right-border-menu {
width: calc(5vw + 47px);
height: 2000px;
background: orange;
float: right;
border-left: 4px solid black;
}