body {
    margin: 0;
    padding: 0;
}
.top,
.bottom {
    height: 350px;
}
.top {
    background-color: #e0e0e0;
}
.bottom {
    background-color: #333;
    position: relative;
}
.buttons {
    margin: 2rem;
}
/* This is all you need */
.progress {
    position: absolute;
    background-color: #333;
    width: 0;
    height: 5px;
    transition: width 1s ease,
                height .2s ease;
}

body {
    margin: 0;
    padding: 0;
}
.top,
.bottom {
    height: 350px;
}
.top {
    background-color: #e0e0e0;
}
.bottom {
    background-color: #333;
    position: relative;
}
.buttons {
    margin: 2rem;
}
/* This is all you need */
.progress {
    position: absolute;
    background-color: #333;
    width: 0;
    height: 5px;
    z-index: 9999;
    transition: width 1s ease,
                height .2s ease;
}

Add z-index to progress CSS
Add z-index to progress CSS
