test: characterize hook observer lifecycle

[thebuilder]

Summary

• characterize useInView lifecycle behavior, observer churn, callback freshness, target movement, skip handling, and Strict Mode cleanup
• characterize useOnInView render behavior, option changes, ref replacement, idempotent cleanup, and diagnostics
• record the current extra useInView attachment commit as the single baseline expected to change in the follow-up refactor

This is the test-only prerequisite for the shared callback-ref observer refactor tracked in #758. It intentionally does not close that issue.

Verification

• corepack pnpm exec vitest run src/__tests__/useInView.test.tsx src/__tests__/useOnInView.test.tsx
• corepack pnpm exec vitest run
• corepack pnpm exec tsc --noEmit
• corepack pnpm exec biome check src/__tests__/useInView.test.tsx src/__tests__/useOnInView.test.tsx src/test-utils.ts
• React 17, 18, and 19 hook tests and typechecks in disposable worktrees

[bolt-new-by-stackblitz]

Run & review this pull request in StackBlitz Codeflow.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
react-intersection-observer	Ready	Preview, Comment	Jul 14, 2026 11:38am

[pkg-pr-new]

npm i https://pkg.pr.new/react-intersection-observer@764

commit: 2f9974b