Using Stylus and adding this CSS will get you a responsive UI for WhatsApp Web (which I think is long overdue, come on Meta).
@media (max-width: 747px){
#app>div>div:last-child>header {
display: none;
}
div:has( > #side):not(:hover) {
max-width: 80px !important;
}
#side div:nth-child(2) {
flex-wrap: nowrap !important;
}
#app>div>div:last-child{
min-width:unset !important;
}
#app>div>div:last-child>div:has( > header > header) > header> header {
display: none !important;
}
div:has( > #side):not(:hover) [role=gridcell]:has([aria-label~="no"]), div:has( > #side):not(:hover) [role=gridcell]:has([aria-label~="No"]) {
display: block;
position: relative;
left: -36px;
}
[data-icon="pinned2"] {
display:none;
}
}