Add react/jsx-runtime and react/jsx-dev-runtime for the new JSX transform. (@lunaruan in #18299)
Build component stacks from native error frames. (@sebmarkbage in #18561)
Allow to specify displayName on context for improved stacks. (@eps1lon in #18224)
Prevent 'use strict' from leaking in the UMD bundles. (@koba04 in #19614)
Stop using fb.me for redirects. (@cylim in #19598)

React DOM

Delegate events to roots instead of document. (@trueadm in #18195 and others)
Clean up all effects before running any next effects. (@bvaughn in #17947)
Run useEffect cleanup functions asynchronously. (@bvaughn in #17925)
Use browser focusin and focusout for onFocus and onBlur. (@trueadm in #19186)
Make all Capture events use the browser capture phase. (@trueadm in #19221)
Don't emulate bubbling of the onScroll event. (@gaearon in #19464)
Throw if forwardRef or memo component returns undefined. (@gaearon in #19550)
Remove event pooling. (@trueadm in #18969)
Stop exposing internals that won’t be needed by React Native Web. (@necolas in #18483)
Attach all known event listeners when the root mounts. (@gaearon in #19659)
Disable console in the second render pass of DEV mode double render. (@sebmarkbage in #18547)
Deprecate the undocumented and misleading ReactTestUtils.SimulateNative API. (@gaearon in #13407)
Rename private field names used in the internals. (@gaearon in #18377)
Don't call User Timing API in development. (@gaearon in #18417)
Disable console during the repeated render in Strict Mode. (@sebmarkbage in #18547)
In Strict Mode, double-render components without Hooks too. (@eps1lon in #18430)
Allow calling ReactDOM.flushSync during lifecycle methods (but warn). (@sebmarkbage in #18759)
Add the code property to the keyboard event objects. (@bl00mber in #18287)
Add the disableRemotePlayback property for video elements. (@tombrowndev in #18619)
Add the enterKeyHint property for input elements. (@eps1lon in #18634)
Warn when no value is provided to <Context.Provider>. (@charlie1404 in #19054)
Warn when memo or forwardRef components return undefined. (@bvaughn in #19550)
Improve the error message for invalid updates. (@JoviDeCroock in #18316)
Exclude forwardRef and memo from stack frames. (@sebmarkbage in #18559)
Improve the error message when switching between controlled and uncontrolled inputs. (@vcarl in #17070)
Keep onTouchStart, onTouchMove, and onWheel passive. (@gaearon in #19654)
Fix setState hanging in development inside a closed iframe. (@gaearon in #19220)
Fix rendering bailout for lazy components with defaultProps. (@jddxf in #18539)
Fix a false positive warning when dangerouslySetInnerHTML is undefined. (@eps1lon in #18676)
Fix Test Utils with non-standard require implementation. (@just-boris in #18632)
Fix onBeforeInput reporting an incorrect event.type. (@eps1lon in #19561)
Fix event.relatedTarget reported as undefined in Firefox. (@claytercek in #19607)
Fix "unspecified error" in IE11. (@hemakshis in #19664)
Fix rendering into a shadow root. (@Jack-Works in #15894)
Fix movementX/Y polyfill with capture events. (@gaearon in #19672)
Use delegation for onSubmit and onReset events. (@gaearon in #19333)
Improve memory usage. (@trueadm in #18970)

React DOM Server

Make useCallback behavior consistent with useMemo for the server renderer. (@alexmckenley in #18783)
Fix state leaking when a function component throws. (@pmaccart in #19212)

React Test Renderer

Improve findByType error message. (@henryqdineen in #17439)

Concurrent Mode (Experimental)

Revamp the priority batching heuristics. (@acdlite in #18796)
Add the unstable_ prefix before the experimental APIs. (@acdlite in #18825)
Remove unstable_discreteUpdates and unstable_flushDiscreteUpdates. (@trueadm in #18825)
Remove the timeoutMs argument. (@acdlite in #19703)
Disable <div hidden /> prerendering in favor of a different future API. (@acdlite in #18917)
Add unstable_expectedLoadTime to Suspense for CPU-bound trees. (@acdlite in #19936)
Add an experimental unstable_useOpaqueIdentifier Hook. (@lunaruan in #17322)
Add an experimental unstable_startTransition API. (@rickhanlonii in #19696)
Using act in the test renderer no longer flushes Suspense fallbacks. (@acdlite in #18596)
Use global render timeout for CPU Suspense. (@sebmarkbage in #19643)
Clear the existing root content before mounting. (@bvaughn in #18730)
Fix a bug with error boundaries. (@acdlite in #18265)
Fix a bug causing dropped updates in a suspended tree. (@acdlite in #18384 and #18457)
Fix a bug causing dropped render phase updates. (@acdlite in #18537)
Fix a bug in SuspenseList. (@sebmarkbage in #18412)
Fix a bug causing Suspense fallback to show too early. (@acdlite in #18411)
Fix a bug with class components inside SuspenseList. (@sebmarkbage in #18448)
Fix a bug with inputs that may cause updates to be dropped. (@jddxf in #18515 and @acdlite in #18535)
Fix a bug causing Suspense fallback to get stuck. (@acdlite in #18663)
Don't cut off the tail of a SuspenseList if hydrating. (@sebmarkbage in #18854)
Fix a bug in useMutableSource that may happen when getSnapshot changes. (@bvaughn in #18297)
Fix a tearing bug in useMutableSource. (@bvaughn in #18912)
Warn if calling setState outside of render but before commit. (@sebmarkbage in #18838)