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The React-18 web-component runtime

The web-component runtime (packages/web-components) runs react/react-dom 18.3.1 on the createRoot API. The host runs React 19 and is not involved. React 18 is the ceiling for shim-free legacy hosting: React 19 removes exactly what the legacy dists stand on (findDOMNode for react-select@2, legacy context for react-bootstrap, ReactDOM.render, the react.element symbol shape, @react-pdf 3's renderer internals). React 19 for these widgets is the rebuild wave (UPSTREAM entry 8), not a runtime swap.

Current state

  • packages/web-components/package.json pins react/react-dom 18.3.1 exact.
  • The react-star-ratings>react override is 18.3.1 in pnpm-workspace.yaml and in host-side override blocks (overrides shadow, they don't merge).
  • The runtime's react entry is plain — no back-fills. React 18 natively carries useSyncExternalStore and useId, the two APIs react-toastify and react-content-loader need (UPSTREAM entry 18). The use-sync-external-store dependency stays: uicore's served chunks import its shim entries.
  • The MUI pin token is pin:mui5 (mui5PinPlugin) — it pins bundles to this package's MUI 5 tree; its mechanics don't depend on the React version.
  • Each element holds one React root per connected span of a visit: mount()'s first render creates it lazily, setProps renders on it, unmount() and disconnectedCallback unmount it (an unmounted 18 root cannot be reused — a reconnect or a new visit creates a fresh one, with the guard making the disconnect-then-unmount removal sequence a no-op second time). Commits are asynchronous and widget-internal setState batches automatically everywhere; nothing host-side reads the DOM after setProps, and readiness rides the widget-painted event — announced by a FirstCommitSignal wrapper (useLayoutEffect) placed below the error boundary, so a failed first render announces nothing.
  • react-dom/client is served through the import map alongside react and react-dom.
  • Dev builds still log deprecations from INSIDE the legacy dists — react-bootstrap's OverlayTrigger renders its overlays through legacy sub-root APIs (unstable_renderSubtreeIntoContainer), warning per overlay render in four widgets (schedule-full/lite, speakers, live-event). Production bundles are silent; those calls retire with the React-19 widget rebuild (UPSTREAM entry 8).

Compatibility inventory (evidence-checked against installed dists)

Fleet-wide: zero React.version checks (except @react-pdf 4's reconciler picker, which takes the same branch for 17 and 18), zero __SECRET_INTERNALS in widget dists proper, zero flushSync, zero string refs, no React-19-only API references.

Dependency Version React-18 verdict
react-select 2.4.4 OK — findDOMNode + childContextTypes exist in 18 (die in 19)
react-bootstrap 0.33.1 OK — legacy context, findDOMNode, unstable_renderSubtreeIntoContainer all functional in 18
react-redux 7.2.9 OK — peers ^16.8 || ^17 || ^18; batches via unstable_batchedUpdates
MUI + @mui/base 5.18.0 / 5.0.0-alpha.127 OK — peers ^17 || ^18 || ^19; useId util is guarded
emotion 11.14 OK — native useInsertionEffect on 18
openstack-uicore-foundation 4.2.x OK — uses the guarded use-sync-external-store shim entries
react-toastify 11.1.0 satisfied — its unguarded useSyncExternalStore is native on 18
react-content-loader ≥7.1 satisfied — its unguarded useId is native on 18
my-orders-tickets-widget 1.0.18 satisfied — peers react ^18.2.0
@react-pdf/renderer 3.4.5 (schedule-full) OK — reads renderer internals that exist in 18 (hard 19 blocker later)
@react-pdf/renderer 4.9.0 (my-tickets) OK — reconciler picker: major <= 18 → same branch as 17
react-stars 2.2.5 OK — createClass + unprefixed lifecycles: dev-only warnings
react-slick, pure-react-carousel, react-tooltip 3, react-spring, Stripe 3/7, formik, react-hook-form, final-form, history 4, moment, redux family, misc hooks libs — OK — React-agnostic or peers cover 18
react-star-ratings 2.3.0 OK — rides the react-star-ratings>react 18.3.1 override (UPSTREAM entry 17)

No widget is riskier on 18 than on 17; the dev-only 18.3 deprecation warnings (defaultProps, legacy context) inventory the eventual React-19 rebuild.

Toward React 19

The runtime is as far as legacy hosting goes: the remaining React-19 blockers live in the widget dists themselves (findDOMNode, legacy context, the @react-pdf 3 internals) and retire with the widget rebuild (UPSTREAM entry 8). The uicore 5.x port is unaffected (5.x peers react ^17 — no conflict, no duplication).