[DevTools] Disable Activity slices by default

[everettbu]

Mirror of facebook/react#35685
Original author: eps1lon

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This feature needs some more work before it's ready to be released. Should've been flagged from the start.

Flag is turned on for local development.

[greptile-apps]

Greptile Overview

Greptile Summary

This PR disables Activity slices in React DevTools by default while keeping them enabled for local development. The new enableActivitySlices feature flag is:

• Set to false for all Facebook builds (core and extension)
• Set to __DEV__ for open-source builds (enabled only in development mode)

The flag properly gates the Activity list UI in SuspenseTab.js, hiding both the Activity panel and its toggle button when the flag is disabled or when no Activity boundaries exist.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The changes are straightforward feature flag additions that properly gate an experimental UI feature, with consistent implementation across all DevTools build configurations
• No files require special attention

Important Files Changed

Filename	Overview
packages/react-devtools-shared/src/config/DevToolsFeatureFlags.default.js	Added enableActivitySlices flag set to __DEV__ for default builds (enabled only in development)
packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseTab.js	Imported and used enableActivitySlices flag to conditionally disable Activity slices UI when flag is off

[greptile-apps]

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[everettbu]

Upstream PR was closed or merged. Code is synced via branch mirror.