Wrapper

Using flexboxes for this purpose is a good solution, however, further css must be implemented in order for the content to be responsive.
An alternative would be to use a wrapper containing both items, and giving it a "position:relative". Then using "position: absolute" in both of your divs (assuming you've got one for the bar on top and anotherone for the content). You would also need to restrict the heights of the top-bar component.

FOR EXAMPLE:

#wrapper{
position: relative;
}
#top-bar, #content{
position: absolute;
}
#top-bar{
height: 50px;
width: 100%;
}
#content{
width:100%;
}