Bootstrap4 fixed-top full-width navbar with non-full-width dropdown-menu - drop-down-menu

Question

I have a navbar with center aligned menu items, of which some are dropdown-menus. The navbar and dropdown menu are 100% width but I would like the drop-down menu to be say 60% width and still centre aligned.
HTML:
<div class="container">
<div class="row">
<div class="col-12">
<nav class="navbar fixed-top navbar-expand-lg navbar-dark" style="background-color: #8B0000;">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent2" aria-controls="navbarSupportedContent2" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent2">
<ul class="navbar-nav mx-auto">
<li class="nav-item dropdown megamenu-li">
<a href="#" class="nav-link dropdown-toggle" aria-haspopup="true" id="navitem-1" data-toggle="dropdown">
Drop down
</a>
<div class="dropdown-menu megamenu" aria-labelledby="navitem-1">
<div class="row">
<div class="col-sm-6 col-lg-3">
<a class="dropdown-item" href="#">Item</a>
<a class="dropdown-item" href="#">Item</a>
</div>
<div class="col-sm-6 col-lg-3">
<a class="dropdown-item" href="#">Item</a>
<a class="dropdown-item" href="#">Item</a>
</div>
</div>
</div>
</li>
<li class="nav-item">
<a href="#" class="nav-link">
Item
</a>
</li>
<li class="nav-item">
<a href="#" class="nav-link">
Item
</a>
</li>
</ul>
</div>
</nav>
</div>
</div>
</div>
CSS :
.megamenu-li {
position: static;
}
.megamenu {
position: absolute;
width: 100%;
left: 0;
right: 0;
padding: 15px;
}
Please see demo here: https://jsfiddle.net/stackoverflow123/53fn2kop/
Reducing the .megamenu width reduces the downdown-menu panel as expected, but it stops it being centrally aligned.
Thank you in advance
desired layout

Answer 1

I managed to approach your goal (if I understood right) applying mainly the next style to the dropdown-menu, but only on the specific bootstrap break-point (LG), and removing all of your style:
.custom-dropdown {
position: fixed !important;
width: 60vw;
left: 20vw !important;
top: 45px !important;
}
Also, I removed the outer row and col-12 and replaced the container by container-fluid, and make some minor changes to the items on the dropdown-menu so they look centered. You can check it below:
#media (min-width: 992px)
{
.custom-dropdown {
position: fixed !important;
width: 60vw;
left: 20vw !important;
top: 45px !important;
}
}
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.6/umd/popper.min.js" integrity="sha384-wHAiFfRlMFy6i5SRaxvfOCifBUQy1xHdJ/yoi7FRNXMRBu5WHdZYu1hA6ZOblgut" crossorigin="anonymous"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css" integrity="sha384-GJzZqFGwb1QTTN6wy59ffF1BuGJpLSa9DkKMp0DgiMDm4iYMj70gZWKYbI706tWS" crossorigin="anonymous">
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/js/bootstrap.min.js" integrity="sha384-B0UglyR+jN6CkvvICOB2joaf5I4l3gm9GU6Hc1og6Ls7i6U/mkkaduKaBhlAXv9k" crossorigin="anonymous"></script>
<div class="container-fluid">
<nav class="navbar fixed-top navbar-expand-lg navbar-dark" style="background-color: #8B0000;">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent2" aria-controls="navbarSupportedContent2" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent2">
<ul class="navbar-nav mx-auto">
<li class="nav-item dropdown">
<a href="#" class="nav-link dropdown-toggle" aria-haspopup="true" id="navitem-1" data-toggle="dropdown">
Drop down
</a>
<div class="dropdown-menu custom-dropdown" aria-labelledby="navitem-1">
<div class="row">
<div class="col-6 text-center">
<a class="dropdown-item" href="#">Item</a>
<a class="dropdown-item" href="#">Item</a>
</div>
<div class="col-6 text-center">
<a class="dropdown-item" href="#">Item</a>
<a class="dropdown-item" href="#">Item</a>
</div>
</div>
</div>
</li>
<li class="nav-item dropdown">
<a href="#" class="nav-link dropdown-toggle" aria-haspopup="true" id="navitem-2" data-toggle="dropdown">
Drop down 2
</a>
<div class="dropdown-menu custom-dropdown" aria-labelledby="navitem-2">
<div class="row">
<div class="col-6 text-center">
<a class="dropdown-item" href="#">Item</a>
<a class="dropdown-item" href="#">Item</a>
</div>
<div class="col-6 text-center">
<a class="dropdown-item" href="#">Item</a>
<a class="dropdown-item" href="#">Item</a>
</div>
</div>
</div>
</li>
<li class="nav-item">
Item
</li>
<li class="nav-item">
Item
</li>
<li class="nav-item">
Item 333
</li>
<li class="nav-item">
Item 99999
</li>
</ul>
</div>
</nav>
</div>

Answer 2

Building off of #Shidersz approach, I wanted a full-width centered nav bar with columns. I used bootstrap 4 and the added a custom-dropdown class. This requires the navbar to be fixed-top or scrolling doesn't have the expected behavior.
.custom-dropdown {
position: fixed !important;
width: 100vw;
left: 0vw !important;
top: 50px !important;
}
<nav class="navbar fixed-top navbar-expand-sm navbar-toggleable-sm navbar-light bg-white border-bottom box-shadow mb-5">
<div class="container">
<a class="navbar-brand">Full Width Awesomness</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target=".navbar-collapse" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="navbar-collapse collapse d-sm-inline-flex flex-sm-row-reverse">
<ul class="navbar-nav flex-grow-1">
<li class="nav-item">
<a class="nav-link text-dark">Top Sellers</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Products
</a>
<div class="dropdown-menu custom-dropdown border-0 border-bottom text-center" aria-labelledby="navbarDropdown">
<div class="row">
<div class="col-sm-3">
<label class="font-weight-bold">Item 1 </label>
<a class="dropdown-item" href="#">Top Sellers</a>
<a class="dropdown-item" href="#">Hot and New</a>
<a class="dropdown-item" href="#">Link 3</a>
</div>
<div class="col-sm-3">
<label>DVDs </label>
<a class="dropdown-item" href="#">Link 1</a>
<a class="dropdown-item" href="#">Link 2</a>
<a class="dropdown-item" href="#">Link 3</a>
</div>
<div class="col-sm-3">
<label>Books </label>
<a class="dropdown-item" href="#">Link 1</a>
<a class="dropdown-item" href="#">Link 2</a>
<a class="dropdown-item" href="#">Link 3</a>
</div>
<div class="col-sm-3">
<label>CDs </label>
<a class="dropdown-item" href="#">Link 1</a>
<a class="dropdown-item" href="#">Link 2</a>
<a class="dropdown-item" href="#">Link 3</a>
</div>
</div>
</div>
</li>
</ul>
</div>
</div>
</nav>
<div class="container-fluid mt-2 pt-5">
<div class="row" style="background-color: pink; height:800px;">
CONTENT
</div>
</div>
fiddle here:
https://jsfiddle.net/mgervasoni/71c0yub4/24/