Replace vendored floating-ui in FloatingDOMController with CSS Anchor Positioning

Start with FloatingDOMController and its vendored floating-ui code, focusing on the affected uses of the global window and the iframe failure described in RedHat-UX/red-hat-design-system#2969. Review the proposed @supports-gated CSS anchor positioning and browser timeline, then verify the iframe fix and the native positioning path against the existing behavior before removing the JavaScript path after October 2026.

Summary

CSS Anchor Positioning reached Baseline in January 2026 (Chrome 125+, Safari 26+, Firefox 147+). The ~1400 lines of vendored floating-ui code in FloatingDOMController can be replaced with native popover attribute + CSS anchor positioning.

Current problems with the vendored code

The vendored floating-ui has diverged from upstream. Several functions use the bare global window where upstream uses getWindow(node) (i.e. node.ownerDocument.defaultView). This causes infinite recursion and stack overflow when components using FloatingDOMController are rendered inside same-origin iframes (see RedHat-UX/red-hat-design-system#2969).

What CSS Anchor Positioning replaces

Timeline

October 2026 is the earliest clean cutover for projects supporting Firefox ESR.

Polyfill considerations

The @oddbird/css-anchor-positioning polyfill (~8KB gzipped) exists but has limitations that make it unsuitable for component libraries:

• Cannot work with constructed stylesheets (how Lit delivers CSS)
• Only processes styles present at call time
• Cross-shadow-root anchoring unsupported
• Global side effects even in "ponyfill" mode

Migration approach

1. Fix the iframe window bug now (one-line fix per affected function)
2. Keep JS positioning as the engine through ESR 140's lifetime
3. Write CSS anchor positioning alongside, gated by @supports (anchor-name: --a)
4. Drop JS positioning path after Oct 2026

References

• CSS Anchor Positioning browser support
• OddBird polyfill shadow DOM limitations
• Web components + anchor positioning + popover
• Firefox ESR schedule
• Baseline status tracking