perf(autosize): fix maxHeight regression under min-height CSS; add Vitest browser test suite

[Copilot]

The performance rewrite on this branch introduced a subtle maxHeight bug by reusing the library's last-written inline height instead of the live computed height. Under min-height/max-height CSS, these diverge — e.g. the library writes "40px" but the element renders at 100px — producing a smaller, incorrect maxHeight cap. This PR restores the original semantics and adds a real-browser test suite to prevent regressions.

src/index.js — maxHeight fix

// Before: uses last-written inline height (misses min-height clamping)
const parsedHeight = height !== null ? parseFloat(height) : parseFloat(getComputedStyle(textarea).height)

// After: always reads rendered height so maxHeight reflects actual layout
const parsedHeight = parseFloat(getComputedStyle(textarea).height)

The cachedBorderAddOn optimization, height tracking for ResizeObserver, and the reads-before-writes batching structure are all preserved.

Test suite — Vitest browser mode

jsdom is a non-starter here (scrollHeight always returns 0, no layout). Tests run in real Chromium via Playwright.

• vitest.config.js — browser mode, playwright provider, chromium, headless
• test/autosize.test.js — 7 tests covering:
  • Grows on input / shrinks on delete
  • min-height floor is respected (rendered height ≥ min-height)
  • Task 1 regression guard: with min-height set, style.maxHeight is not capped below the live computed height
  • Drag-resize latches isUserResized and clears maxHeight
  • Form reset re-enables autosize
  • unsubscribe() stops further resizing

Supporting changes

• devDependencies: vitest@4.1.9, @vitest/browser@4.1.9, @vitest/browser-playwright@4.1.9, playwright@1.61.1 (all at patched versions per advisory DB check)
• package.json "test" script replaced with vitest run; "test:watch" added; pretest build hook removed (tests import src/index.js directly, no build needed)
• tsconfig.json: skipLibCheck: true to prevent vitest's @types/chai conflicting with the existing target: es5 build
• .gitignore: exclude test/__screenshots__/

[Copilot]

Pull request overview

This PR fixes a maxHeight regression in autosize when min-height/max-height CSS causes the rendered height to diverge from the library’s last-written inline height, and it introduces a real-browser (Chromium/Playwright) Vitest test suite to prevent future regressions.

Changes:

• Restore original maxHeight semantics by always deriving it from the live computed (rendered) textarea height.
• Add Vitest browser-mode configuration and a new browser-based test suite covering growth/shrink, min-height behavior, drag-resize disabling, form reset, and unsubscribe cleanup.
• Update tooling/config (package.json scripts + devDependencies, tsconfig.json skipLibCheck, .gitignore) to support the new test setup.

Show a summary per file

File	Description
src/index.js	Fixes maxHeight calculation to use rendered/computed height under CSS clamping.
test/autosize.test.js	Adds Chromium-based regression and behavior tests for autosizing and resize detection.
vitest.config.js	Enables Vitest browser mode using the Playwright provider (Chromium headless).
package.json	Replaces placeholder test script with vitest run and adds Vitest/Playwright devDependencies.
package-lock.json	Locks the new Vitest/Playwright dependency graph.
tsconfig.json	Adds skipLibCheck to avoid type-check conflicts introduced by new test tooling types.
.gitignore	Ignores Vitest browser screenshot output directory.

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• Files reviewed: 4/8 changed files
• Comments generated: 2
• Review effort level: Low