style.css

body {
    margin: 0;
    overflow: hidden;
}

.container  {
    display: flex;
    width: calc(100vw + 400px - 40px);
    justify-content: space-between;
    position: relative;
    left: -180px;
    transition: left 200ms ease-in-out;
}

.content {
    flex-grow: 2;
    background-color: blue;
    display: flex;
    justify-content: center;
    align-items: center;
}

.inner-content {
    background-color: gray;
    width: calc(100vw - 80px);
    height: calc(100vh - 40px);
}

.side-panel {
    height: 100vh;
    background-color: orange;
    width: 200px;
    z-index: 2;
    box-shadow: 0 0 10px 3px rgba(0, 0, 0, .5);
    transition: background-color 200ms ease-in-out;
}

.side-panel:hover {
    background-color: #ffbe00;
    cursor: pointer;
}