React 19 support, migrate to @react-spring/web and @use-gesture/react

[Guiw5]

Summary

Migrates the sheet off react-spring v8 / react-use-gesture (both unmaintained, and v8 breaks under React 18/19 StrictMode: it destroys its controllers on the dev double-mount and never recreates them) onto @react-spring/web 10 and @use-gesture/react 10, and adds React 19 to the peer deps. Also fixes several regressions from the XState v5 migration.

Fixes

• skipInitialTransition was ignored: the machine never received initialState, now passed via input.
• data-rsbs-state was only set for atomic states (current.value === 'opening' is false for nested values), so the opening/closing CSS never applied. Uses matches() now.
• console.log/console.count in every render, in useReady, in the focus trap, plus debug actors with sleep(1000) shipped in the bundle.
• onSpringStart / onSpringEnd promises were not awaited, so returning a promise no longer delayed the transition.
• Operator precedence bug in useMaxHeight initial state.
• SpringConfig required every key; it is now Partial.
• NODE_ENV=production in the build script did not run on Windows (cross-env).
• postcss-svgo was a runtime dependency; moved to devDependencies.

Performance

• Machine transitions no longer re-render React: only the data-rsbs-state attribute depended on them, so it is written to the DOM from an actor subscription (useActorRef). Opening previously triggered ~8 renders mid-animation.
• Machine implementations and spring interpolations are created once instead of on every render.
• Dropped @juggle/resize-observer: it polyfills even when the native API exists and recomputes on every DOM mutation. Native ResizeObserver with a borderBoxSize fallback.
• Modern browserslist (native ResizeObserver baseline) so less transpilation output.

Compatibility

• Peer deps: react and react-dom ^16.14 || ^17 || ^18 || ^19.
• Types compile against both @types/react 18 and 19 (no global JSX, no useRef() without argument, no MutableRefObject).
• aria-modal follows blocking; closed-but-mounted sheets (keepMounted) get visibility: hidden so they leave the tab order and a11y tree.
• Docs site runs with reactStrictMode: true so the v8 StrictMode bug cannot regress silently.

Behaviour note

The published version animates with a 115ms linear tween (duration: 115 overrides tension/friction/velocity). Kept as the default; springConfig={{ duration: undefined }} restores spring physics. Documented on SpringConfig.

Numbers

	before	after
Bundle incl. deps (gzip)	39.4 kB	49.9 kB (react-spring v10 is ~2x v8)
Frames > 33ms open/drag/close, 4x CPU throttle	n/a	3 of 198, p95 16.9ms

Test plan

• npm test (library build, declarations, Next docs build)
• tsc with @types/react 18 and 19, eslint
• Chrome (mobile emulation, StrictMode): open/close, Escape dismiss, header drag with snap, multi snap points, content resize, scroll lock and aria-hidden restore
• Real iOS Safari and Android Chrome

[github-actions]

🎉 This PR is included in version 3.9.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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