Bug - Tabs - Tabs overflow does not respond to container resize (only listens to window.resize)

Description

The Tabs component only recalculates its overflow state (scroll buttons and "More" dropdown count) in response to window.resize events. It does not observe its own container for size changes. This means that when Tabs are placed inside any dynamically-sized container — such as a resizable Drawer, a CSS Grid/Flexbox layout that redistributes space, or a collapsible sidebar — the overflow state becomes stale:

• Scroll buttons appear when all tabs fit, or disappear when tabs overflow

• The "More (N)" dropdown count does not update

• The active tab accent bar drifts out of position

Root Cause

In Tabs.tsx, componentDidMount (line 381) registers only:

window.addEventListener('resize', this.handleResize, false);

There is no ResizeObserver on this.tabList.current (the <ul> element), so container-level width changes are invisible to the component.

Additionally, handleScrollButtons() (line 258) uses a 100ms debounce and componentDidUpdate (line 429) accumulates overflowingTabCount rather than recalculating from scratch, which can compound errors during rapid resizes.

Steps to Reproduce

1. Place a <Tabs> component with 8+ tabs inside a <DrawerPanelContent isResizable>
2. Open the drawer — tabs may display correctly initially
3. Drag the drawer splitter to make the panel narrower
4. Expected: Scroll buttons appear / "More" count increases as tabs overflow
5. Actual: Overflow state is unchanged until the browser window itself is resized

This also reproduces with any non-window container resize (e.g., a parent element resized via JavaScript, CSS transitions, or layout shifts).

Proposed Fix

Add a ResizeObserver on this.tabList.current in componentDidMount, alongside the existing window.resize listener. The observer should filter by width changes only — when scroll buttons render/hide, they change the container height, which would re-trigger the observer and create a feedback loop.

// New instance property
private resizeObserver: ResizeObserver | null = null;
private lastObservedWidth: number = 0;

componentDidMount() {
  if (!this.props.isVertical) {
    if (canUseDOM) {
      window.addEventListener('resize', this.handleResize, false);

      // Observe container size changes (e.g., inside resizable Drawer)
      this.resizeObserver = new ResizeObserver((entries) => {
        const newWidth = entries[0].contentRect.width;
        // Only react to WIDTH changes — scroll button rendering changes height,
        // which would cause an infinite feedback loop if not filtered out.
        if (Math.abs(newWidth - this.lastObservedWidth) < 1) {
          return;
        }
        this.lastObservedWidth = newWidth;
        this.handleResize();
      });
      if (this.tabList.current) {
        this.resizeObserver.observe(this.tabList.current);
      }
    }
    this.direction = getLanguageDirection(this.tabList.current);
    this.handleScrollButtons();
  }
  this.setAccentStyles(true);
}

componentWillUnmount() {
  if (!this.props.isVertical) {
    if (canUseDOM) {
      window.removeEventListener('resize', this.handleResize, false);
      this.resizeObserver?.disconnect();
    }
  }
  // ...existing cleanup
}

This would fix Tabs in any dynamically-sized container, not just Drawer.

Current Workaround

Consumers can work around this by attaching their own ResizeObserver and forcing a Tabs remount via a key prop. However, the workaround has three non-obvious pitfalls:

6. Must use a callback ref, not useRef + useEffect — DrawerPanelContent delays rendering children until a CSS transition completes (isExpandedInternal is set on transitionEnd). A useEffect keyed on isExpanded runs before the panel DOM node exists, so the observer never attaches after a drawer toggle.

7. Must filter by width changes only — When scroll buttons appear/disappear, they change the container height. A naive ResizeObserver callback re-triggers on height changes, causing an infinite remount loop (buttons render → height changes → observer fires → remount → repeat).

8. Must debounce to ~150ms — Tabs internally uses a 100ms debounce in handleScrollButtons() and a 100ms delay for showScrollButtons. The consumer's debounce must be longer than these to avoid interfering with the multi-phase scroll button rendering cycle.

const [resizeKey, setResizeKey] = useState(0);
const lastWidthRef = useRef<number>(0);
const observerRef = useRef<ResizeObserver | null>(null);
const debounceRef = useRef<ReturnType<typeof setTimeout>>();

const panelCallbackRef = useCallback((node: HTMLDivElement | null) => {
  clearTimeout(debounceRef.current);
  observerRef.current?.disconnect();
  observerRef.current = null;
  if (!node) return;

  lastWidthRef.current = 0;
  observerRef.current = new ResizeObserver((entries) => {
    const newWidth = entries[0].contentRect.width;
    if (Math.abs(newWidth - lastWidthRef.current) < 1) return;
    lastWidthRef.current = newWidth;
    clearTimeout(debounceRef.current);
    debounceRef.current = setTimeout(() => setResizeKey(c => c + 1), 150);
  });
  observerRef.current.observe(node);
}, []);

// In your DrawerPanelContent:
<div ref={panelCallbackRef}>
  <Tabs key={resizeKey} activeKey={activeKey} onSelect={handleSelect}>
    {/* ... */}
  </Tabs>
</div>

Environment

• @patternfly/react-core version: current main

• Browsers: all (ResizeObserver is widely supported)

Related

• Jira: PF-3907

---

Jira Issue: PF-4026

Tabs.tsx で componentDidMount、componentWillUnmount、handleScrollButtons、componentDidUpdate を読み始め、既存の window のリサイズ処理と debounce の動作を追ってください。サイズ変更可能な Drawer またはその他の変化するコンテナで問題を再現し、オーバーフローコントロールと More の件数が、高さだけの変更に繰り返し反応することなく更新されることを確認してください。