Add <React.Profiler> API for gathering performance measurements programmatically. (@bvaughn in #15172)
Remove unstable_ConcurrentMode in favor of unstable_createRoot. (@acdlite in #15532)

Deprecate old names for the UNSAFE_* lifecycle methods. (@bvaughn in #15186 and @threepointone in #16103)
Deprecate javascript: URLs as a common attack surface. (@sebmarkbage in #15047)
Deprecate uncommon "module pattern" (factory) components. (@sebmarkbage in #15145)
Add support for the disablePictureInPicture attribute on <video>. (@eek in #15334)
Add support for onLoad event for <embed>. (@cherniavskii in #15614)
Add support for editing useState state from DevTools. (@bvaughn in #14906)
Add support for toggling Suspense from DevTools. (@gaearon in #15232)
Warn when setState is called from useEffect, creating a loop. (@gaearon in #15180)
Fix a memory leak. (@paulshen in #16115)
Fix a crash inside findDOMNode for components wrapped in <Suspense>. (@acdlite in #15312)
Fix pending effects from being flushed too late. (@acdlite in #15650)
Fix incorrect argument order in a warning message. (@brickspert in #15345)
Fix hiding Suspense fallback nodes when there is an !important style. (@acdlite in #15861 and #15882)
Slightly improve hydration performance. (@bmeurer in #15998)

Fix incorrect output for camelCase custom CSS property names. (@bedakb in #16167)

Add act(async () => ...) for testing asynchronous state updates. (@threepointone in #14853)
Add support for nesting act from different renderers. (@threepointone in #16039 and #16042)
Warn in Strict Mode if effects are scheduled outside an act() call. (@threepointone in #15763 and #16041)
Warn when using act from the wrong renderer. (@threepointone in #15756)