ReferenceError: ShadowRoot is not defined when auto-polyfill runs without global ShadowRoot

[steve02081504]

Summary

Importing @oddbird/css-anchor-positioning (e.g. via esm.sh) auto-invokes polyfill() from src/index.ts. In environments that have window/document but do not expose a global ShadowRoot (incomplete DOM shims, some test harnesses), that call rejects with:

ReferenceError: ShadowRoot is not defined
    if (root instanceof ShadowRoot) {

at src/dom.ts (querySelectorAllRoots).

Why this is hard to catch for consumers

1. index.ts calls polyfill() without awaiting or .catch()ing the returned promise, so the failure surfaces as an unhandled rejection, not as a failed import().
2. Wrapping the dynamic import in .catch() therefore does not protect callers — the module evaluates successfully, then the async polyfill blows up later.

Repro sketch

// Any host with document but no global ShadowRoot
globalThis.document = /* minimal document */;
// globalThis.ShadowRoot intentionally unset

await import('@oddbird/css-anchor-positioning');
// → unhandled rejection: ReferenceError: ShadowRoot is not defined

Seen with @oddbird/css-anchor-positioning@0.10.1 under Deno + a partial happy-dom global install (document present, ShadowRoot not copied onto globalThis).

Suggested fixes

1. Guard the check, e.g. typeof ShadowRoot !== 'undefined' && root instanceof ShadowRoot.
2. Catch / await the auto-polyfill() promise in index.ts so a missing DOM feature does not become an unhandled rejection on mere import.
3. Optionally skip auto-apply when required DOM APIs (ShadowRoot, etc.) are absent.

Happy to test a PR if useful.

[steve02081504]

Impact on steve02081504/fount: CI pure markdown tests pulled this package via \js-polyfill\ → \�sm.sh/@oddbird/css-anchor-positioning, then failed on the unhandled \ShadowRoot\ rejection before any test body ran.

Workaround on our side (until this lands): stop importing page bootstrap (\�ase.mjs) from template path-resolution code; expose \ShadowRoot\ on the test DOM shim.

After upstream fix: we can drop the defensive \ShadowRoot\ global in the shim if nothing else needs it; no fount API change required for the polyfill import path itself (still behind .catch).

[steve02081504]

Update (fount): dropped the base_dir.mjs split. base.mjs now (1) mirrors window DOM constructors onto globalThis before loading polyfills, (2) skips polyfill/Sentry/SW when viewport is 0x0, and (3) briefly fences unhandledrejection matching ShadowRoot / css-anchor-positioning while js-polyfill settles. After this issue is fixed, (3) can be removed; (1) remains useful for Deno + happy-dom.

[jamesnw]

@jpzwarte Can you take a look at this?