fix: resolve infinite update loop when using createRoot (React 18)

[zerosrat]

Problem

When using React 18's createRoot (instead of legacy ReactDOM.render), switching between cached KeepAlive components triggers "Maximum update depth exceeded" errors. This makes react-activation incompatible with createRoot.

Related issues: #336, #257

Root Cause

Two code paths create infinite synchronous render loops under createRoot:

1. FallbackListener.componentDidMount (Suspense.js)

componentDidMount calls onStart → setState({ suspense: true }) synchronously, creating a tight loop:

componentDidMount → setState → re-render → throw promise → 
Suspense fallback → componentDidMount → setState → ...

In React 17 / legacy ReactDOM.render, batching behavior prevented this from becoming an infinite loop. With createRoot, React 18's automatic batching works differently for Suspense fallback rendering, making the cycle synchronous and hitting the max update depth.

2. Keeper uses flushSync (Keeper.js)

flushSync forces synchronous rendering inside createRoot, bypassing React 18's automatic batching and amplifying the infinite loop.

Fix

1. Suspense.js: Defer onStart callback to a microtask using Promise.resolve().then(), breaking the synchronous cycle. Added an _unmounted guard to prevent calling setState after unmount.

2. Keeper.js: Remove flushSync wrapper around the freeze setState call. The setTimeout(..., 1000) already provides the necessary delay for post-cache processing.

Testing

Tested with:

• react@18.3.1 + react-dom@18.3.1
• react-activation@0.13.4
• App using createRoot with AliveScope + KeepAlive
• Verified: switching between cached tabs no longer triggers infinite update errors

🤖 Generated with Claude Code