Sliding tabs

<div class="tabs">
  <div class="tab-list" role="tablist" aria-label="Project views">
    <button role="tab" id="tab-overview" aria-controls="panel-overview" aria-selected="true">Overview</button>
    <button role="tab" id="tab-activity" aria-controls="panel-activity" aria-selected="false" tabindex="-1">Activity</button>
    <button role="tab" id="tab-settings" aria-controls="panel-settings" aria-selected="false" tabindex="-1">Settings</button>
    <span class="tab-indicator" aria-hidden="true"></span>
  </div>
  <div role="tabpanel" id="panel-overview" aria-labelledby="tab-overview">Everything at a glance: status, owners and the latest deploy.</div>
  <div role="tabpanel" id="panel-activity" aria-labelledby="tab-activity" hidden>Recent commits, comments and releases, newest first.</div>
  <div role="tabpanel" id="panel-settings" aria-labelledby="tab-settings" hidden>Names, access and notifications for this project.</div>
</div>

.tabs { width: min(440px, 100%); font: 15px/1.5 system-ui, sans-serif; }

.tab-list {
  position: relative;
  display: flex;
  gap: 4px;
  padding: 4px;
  border-radius: 12px;
  background: var(--surface-2);
  box-shadow: inset 0 0 0 1px var(--line);
}
.tab-list [role="tab"] {
  position: relative;
  z-index: 1;               /* above the indicator */
  flex: 1;
  border: 0;
  cursor: pointer;
  padding: 9px 12px;
  border-radius: 9px;
  font: 600 14px system-ui, sans-serif;
  color: var(--muted);
  background: transparent;
  transition: color 0.2s;
}
.tab-list [role="tab"][aria-selected="true"] { color: var(--fg); }
.tab-list [role="tab"]:focus-visible { outline: 2px solid var(--brand); outline-offset: 2px; }

/* The highlight behind the active tab. JavaScript sets its position and width. */
.tab-indicator {
  position: absolute;
  top: 4px;
  bottom: 4px;
  left: 0;
  width: var(--indicator-width, 0);
  transform: translateX(var(--indicator-left, 0));
  border-radius: 9px;
  background: var(--surface);
  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.12), inset 0 0 0 1px var(--line);
  transition: transform 0.35s cubic-bezier(0.22, 1, 0.36, 1), width 0.35s cubic-bezier(0.22, 1, 0.36, 1);
}
@media (prefers-reduced-motion: reduce) { .tab-indicator { transition-duration: 0.15s; } }

[role="tabpanel"] { padding: 16px 6px 0; color: var(--muted); }

// Sliding tabs: follows the WAI-ARIA tabs pattern (arrow keys move between
// tabs, Home/End jump to the ends) and slides an indicator under the active tab.

document.querySelectorAll('.tabs').forEach((tabsRoot) => {
  const tabs = [...tabsRoot.querySelectorAll('[role="tab"]')];
  const indicator = tabsRoot.querySelector('.tab-indicator');

  function moveIndicatorTo(tab) {
    indicator.style.setProperty('--indicator-left', `${tab.offsetLeft}px`);
    indicator.style.setProperty('--indicator-width', `${tab.offsetWidth}px`);
  }

  function selectTab(tabToSelect, { moveFocus = false } = {}) {
    for (const tab of tabs) {
      const isSelected = tab === tabToSelect;
      tab.setAttribute('aria-selected', String(isSelected));
      tab.tabIndex = isSelected ? 0 : -1;  // only the active tab is in the Tab order
      document.getElementById(tab.getAttribute('aria-controls')).hidden = !isSelected;
    }
    moveIndicatorTo(tabToSelect);
    if (moveFocus) tabToSelect.focus();
  }

  tabs.forEach((tab, index) => {
    tab.addEventListener('click', () => selectTab(tab));

    tab.addEventListener('keydown', (event) => {
      const keyToIndex = {
        ArrowRight: (index + 1) % tabs.length,
        ArrowLeft: (index - 1 + tabs.length) % tabs.length,
        Home: 0,
        End: tabs.length - 1,
      };
      if (event.key in keyToIndex) {
        event.preventDefault();
        selectTab(tabs[keyToIndex[event.key]], { moveFocus: true });
      }
    });
  });

  // Place the indicator now, and again whenever the tab sizes change (resize, font load).
  const currentTab = () => tabs.find((tab) => tab.getAttribute('aria-selected') === 'true');
  moveIndicatorTo(currentTab());
  new ResizeObserver(() => moveIndicatorTo(currentTab())).observe(tabsRoot);
});