Edit this CSS:
#post-nav .post-nav-wrap {
background-size: cover !important;
background-position: center center !important;
min-height: 120px;
height: 100%;
padding: calc(6% + 1em) 5%;
-webkit-box-shadow: inset 0 -50px 70px 20px rgba(0, 0, 0, 0.5);
box-shadow: inset 0 -50px 70px 20px rgba(0, 0, 0, 0.5);
-webkit-transition: -webkit-box-shadow 500ms;
transition: -webkit-box-shadow 500ms;
transition: box-shadow 500ms;
transition: box-shadow 500ms, -webkit-box-shadow 500ms;
position: relative;
}
Remove the height: 100%;
property and if need be reduce the min-height: 120px;