header {
height: 80px;
width: 100%;
background: #fff;
position: absolute;
display: block;
z-index: 10;
transition: background-color 0.3s;
}
#Barra-Navegacion {
left: 0;
top: 0;
width: 100%;
position: fixed;
display: block;
margin:10px auto 0;
height: 40px;
transition: margin-left 0.3s;
}

Position: absolute;
z-index: 9999;