Re-enable Activity in config panel

[everettbu]

Mirror of facebook/react#35401
Original author: afurm

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• Summary: Restore <Activity> wrappers around the config panel’s expanded/collapsed views in compiler/apps/playground/components/Editor/ConfigEditor.tsx after the Next.js upgrade.

[greptile-apps]

Greptile Summary

This PR restores the <Activity> component wrappers around the config panel's expanded and collapsed views, which were temporarily disabled during a Next.js downgrade for security reasons. Now that Next.js has been upgraded to version 15.5.9, the <Activity> components can be safely re-enabled.

The change replaces temporary div elements with inline display styles with proper <Activity mode> wrappers that provide better integration with React's view transition system. The implementation correctly uses:

• <Activity mode={isExpanded ? 'visible' : 'hidden'}> for the expanded editor
• <Activity mode={isExpanded ? 'hidden' : 'visible'}> for the collapsed editor

This ensures proper visibility toggling coordinated with the config panel transitions.

Confidence Score: 5/5

• This PR is safe to merge with no identified risks
• The change is a straightforward restoration of previously working code that was temporarily disabled. The Activity component is properly exported from React 19.2.3, Next.js has been upgraded to 15.5.9 (addressing the security concerns that caused the temporary removal), and the implementation follows the exact pattern that was used before. The logic is simple and correct - toggling between two mutually exclusive states with proper visibility controls.
• No files require special attention

Important Files Changed

Filename	Overview
compiler/apps/playground/components/Editor/ConfigEditor.tsx	Restored <Activity> wrappers around expanded/collapsed views after Next.js upgrade, replacing temporary div workaround

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