New Architecture only — the legacy architecture has been removed; apps on the old architecture must migrate to Fabric / the New Architecture: Enable RCT_REMOVE_LEGACY_ARCH (#16157)
WebSocket internals unified — the legacy WinRTWebSocketResource and JSBigStringResourceDll were removed; internal, but custom native forks touching these may need updates: Remove old WinRTWebSocketResource implementation (#16229), Remove JSBigStringResourceDll, and unify WebsocketResource (#16159)

✨ New Features & Enhancements

Adds hideTitleBar and hideBorder props to Modal: Add hideTitleBar and hideBorder to Modal (#16108)
Implements outline style properties on View: Implement outline properties on view (#16074)
Adds onAuxClick event support: Add onAuxClick event (6ca3cc9)
Adds ability to make TurboModules eager init: Add ability to make TurboModules eager init (#16093)
Shows tooltip on keyboard focus and enforces a single visible tooltip: Show tooltip on keyboard focus, enforce single visible tooltip (#16123)
Exposes ReactNativeWindow from ReactNativeWin32App: Expose ReactNativeWindow from ReactNativeWin32App (#16214)

🔧 Reliability & Stability

Text & input - Fixes text input scaling with different screen scales: Fix: text input scaling with different screen scales (#16288) - Fixes Unicode text length calculation: fix: Unicode Text length Calculation (#16219) - Fixes multiline TextInput crash from TxDrawD2D reentrancy: Fix multiline TextInput crash from TxDrawD2D reentrancy (#16003) - Fixes x86 assert when using custom align: Fix assert in x86 when using custom align (#16152)

Touch, pointer & ScrollView - Improves touch event handling reliability and performance: Fix touch event handling, improve reliability, and optimize performance (#16048) - Fixes ScrollView Pressables stuck / dead tap on high-DPI displays: Fix ScrollView Pressables stuck and dead tap on high-DPI displays (#16140) - Fixes ScrollView keyboard scroll drift on long key press: Fix ScrollView keyboard scroll drift on long key press (#15988) - Cancels zombie touch state on ScrollView pointer-capture loss: fix: cancel zombie touch state on ScrollView pointer capture loss (#16100), fix: cancel active touches in onPointerCaptureLost (#16086) - Uses IsPrimary() for touch pointer isPrimary: fix: use IsPrimary() for touch pointer isPrimary instead of hardcoded id (#16099) - Maps Windows touch pointer IDs to small JS-safe identifiers: fix: map Windows touch pointer IDs to small JS-safe identifiers in CompositionEventHandler (#16081)

Rendering & views - Fixes some issues with rounded borders: Fix some issues with rounded borders (#16051) - Fixes Image not consistently re-rendering on source change: Image does not consistently rerender on image source change (#16107) - Fixes null dereference in get_SelectionContainer when no selection container exists: fix: null dereference in get_SelectionContainer (#16091) - Fixes Modal title bar contrast when the native window is inactive: Fix inactive Modal title bar contrast (#16449)

Accessibility - Raises UI Automation selection events when accessibilityState.selected changes: Raise UIA events for selection changes (#16447)

Runtime & stability - Fixes crash attempting to get runtime when shutting down instance: Fix crash attempting to get runtime when shutting down instance (#16238) - Fixes race condition that can cause the callinvoker to get a null runtime: Race condition can cause the callinvoker to get a null runtime (#16186) - Fixes WebSocket binaryType handling for binary messages: fix: WebSocket binaryType handling (#16173)

⚡ Performance

Text rendering performance improvement: Text rendering perf improvement (#16083)

🧰 CLI & Templates

run-windows can start Metro on a custom port: fix: allow run-windows to start Metro on custom port (#16126)
More consistent template config between ship/debug builds: Update template to have more consistent config between ship/debug (#16165)
Template imports Microsoft.Cpp.Default.props before setting ReactNativeWindowsDir: Template should import Microsoft.Cpp.Default.props before setting ReactNativeWindowsDir (#16146)
NativeModule templates work with clang: Fix NativeModule templates to work with clang (#15917)

Also includes routine dependency updates (boost 1.84, hermes-windows, PowerShell 7) and CI/build infrastructure improvements. Full changelog: 0.84-stable...0.85-stable.