<div style={{ flex:1 }}>
    <div class="sideBarContainer">
         <div class="sideBar">
         <div class="sideBarExtra"> <--- conditional
    </div>
    <div class="content"></div>
</div>

.sideBarContainer {
    display: flex;
    flex-direction: row;
}
.sideBar {
    height: 100vh;
    min-width: 64px;
    z-index: 99;
}
.content {
    flex: 1;
    transition: width: .3s linear;
}