[0.85] React Native DevTools "Performance" panel: recording a trace then Stop disconnects the app

Summary

In the Modern debugger (React Native DevTools / fusebox), opening the Performance panel, recording a trace, and pressing Stop reproducibly disconnects the debugging session — the app drops off with "DevTools is disconnected — Connection lost to corresponding device." The app has to be relaunched to reconnect.

Environment

• react-native-windows: 0.85.0-preview.1
• react-native: 0.85.3
• @react-native/dev-middleware: 0.85.3
• @react-native/debugger-frontend: 0.85.3
• JS engine: Hermes
• OS: Windows 11 (10.0.26200)
• Repro app: NativeModuleSample (cpp-lib) example

System: OS: Windows 11 10.0.26200 CPU: (16) x64 Intel(R) Core(TM) i9-9900 CPU @ 3.10GHz Memory: 35.15 GB / 63.84 GB Binaries: Node: 22.22.0 npm: 10.9.4 SDKs: Windows SDK Versions: 10.0.22621.0, 10.0.26100.0 IDEs: Visual Studio: 18.7.12002.237 (Visual Studio Enterprise 2026) npmPackages: react: 19.2.3 react-native: 0.85.3 react-native-windows: 0.85.0-preview.1 @react-native-community/cli: 20.2.0

Steps to reproduce

1. Launch an RNW 0.85 Hermes app and connect Metro.
2. Press j in Metro to open React Native DevTools.
3. Open the Performance tab.
4. Click Record, interact with the app for a couple of seconds.
5. Click Stop.

Expected

A performance timeline / trace renders in the panel.

Actual

The app immediately disconnects: "DevTools is disconnected. Debugging connection was closed. Reason: Connection lost to corresponding device." The trace is lost and the app must be relaunched. Reproduced 2/2 times. A headless CDP probe showed the classic Profiler.stop / trace-stop path also hangs, consistent with this.

Other panels (context — all validated on RNW 0.85)

• Console ✅ • Sources/Breakpoints ✅ (breakpoint actually hits & pauses) • Memory ✅ (heap snapshot) • Components ✅ • Profiler ✅
• Network ⚠️ — panel loads and records, but no requests appear. This is a separate, pre-existing platform gap (RNW's WinRT networking is not wired to the inspector's Network domain / NetworkReporter); it is not specific to 0.85. Documenting as a known limitation.

Root-cause notes (not an RNW-0.85 regression)

• The debugger stack is stock upstream RN 0.85.3 (dev-middleware, debugger-frontend, react-devtools-core unmodified by RNW).
• Diffing 0.84-stable → 0.85-stable, RNW's own inspector/Hermes/DevSupport native code has zero changes (HermesRuntimeAgentDelegate, HermesRuntimeTargetDelegate, ReactInspectorPackagerConnectionDelegate, DevSupportManager).
• RNW has no CDP Tracing code of its own; the Performance/trace path is handled entirely by upstream Hermes + jsinspector-modern.
• Therefore this is upstream Hermes/DevTools behavior surfacing on Windows, not a change introduced by the 0.85 integration. Filed here for tracking / upstream escalation.

Severity

Low-Medium — user-facing (one of seven debugger panels is unusable via record→stop), but does not affect app runtime; only the debugging session drops. Workaround: relaunch the app; use the React Profiler panel for render timing instead.