Resize Observer

<div class="v"><header>RESIZE OBSERVER <span id="width">— px</span></header><div class="stage size"><div class="resizable" id="box"><div class="bars"><i></i><i></i><i></i></div><span id="mode">wide layout</span></div><div class="ruler"></div></div><div class="muted">container width → component layout</div></div>

.v{width:min(94vw,760px);height:min(88vh,320px);padding:clamp(9px,2.4vmin,18px);border:1px solid var(--line);border-radius:12px;background:var(--surface);font:500 clamp(12px,2.4vmin,16px)/1.3 var(--font-sans,system-ui,sans-serif);display:flex;flex-direction:column;gap:clamp(5px,1.4vmin,10px);overflow:hidden}.v header{display:flex;align-items:center;justify-content:space-between;gap:8px;color:var(--accent);font-weight:700}.v header span{color:var(--muted);font-weight:500}.v .stage{flex:1;min-height:0;position:relative}.v .muted{color:var(--muted)}.v .active{background:color-mix(in srgb,var(--accent) 18%,var(--surface));border-color:var(--accent)!important;color:var(--fg)}.v button{font:inherit;color:var(--fg);background:var(--bg);border:1px solid var(--line);border-radius:7px;padding:5px 8px;cursor:pointer}.size{display:flex;align-items:center;justify-content:center;flex-direction:column;gap:5px}.resizable{width:75%;height:68%;border:2px solid var(--accent);border-radius:8px;background:var(--bg);display:flex;align-items:center;justify-content:center;gap:8px;animation:resize 3s ease-in-out infinite alternate}.bars{display:flex;align-items:end;gap:4px;height:35px}.bars i{display:block;width:10px;background:var(--accent);height:50%}.bars i:nth-child(2){height:100%}.bars i:nth-child(3){height:70%}.ruler{width:75%;border-top:1px dashed var(--muted)}@keyframes resize{to{width:42%}}@media(max-width:420px){.resizable{width:82%}@keyframes resize{to{width:52%}}}

const box=document.getElementById("box");new ResizeObserver(([entry])=>{const w=Math.round(entry.contentRect.width);document.getElementById("width").textContent=w+" px";document.getElementById("mode").textContent=w<150?"compact":"wide layout"}).observe(box)