happy-dom v20 vs jsdom on Vitest 4: realistic speedup, the three failure zones, and the per-project migration

As of 2026-08-15: switching a Vitest 4 Testing Library suite from jsdom to happy-dom 20.11.2. Realistic speedup range, the three failure zones to keep on jsdom, the v20 security boundary, and the per-project migration recipe since environmentMatchGlobs was removed. — Read the full piece on Tenjin:…