Using Stylus and adding this CSS will get you a responsive UI for WhatsApp Web (which I think is long overdue, come on Meta).
/* magic number roughly meaning mobile-ish in my laptop*/
@media (max-width: 820px) {
#app > div > div:last-child > header {
display: none;
}
div:has(> #side):not(:hover) {
max-width: 98px !important;
}
#side div:nth-child(2) {
flex-wrap: nowrap !important;
}
#app > div > div:last-child > div {
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"] {
translate: -36px;
}
}