[DevTools] Fix crash when revealing stable, filtered <Activity> children

[everettbu]

Mirror of facebook/react#35734
Original author: eps1lon

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Summary

This aligns the hidden -> visible Activity path with how we treat hidden Activity. Since we don't mount hidden Activity children, we need to switch to the mount path when updating an Activity from hidden to visible.

This mostly went unnoticed since the update that reveals Activity also created fresh children. But for children from props, we continue down the update path and then later crash because children unexpectedly changed.

Originally I added an extra isReconnectingSubtree before realizing the whole transition from hidden to visible wasn't correctly modelled.

How did you test this change?

• Added test
• https://flvrxf.csb.app/ no longer crashes (used to crash with The children should not have changed if we pass in the same set.)

[greptile-apps]

Greptile Overview

Greptile Summary

This PR fixes a DevTools backend reconciliation bug when an Offscreen/Activity boundary transitions from mode="hidden" to mode="visible".

Previously, hidden Activity children were not mounted (and were unmounted when hiding), but the hidden→visible transition could still go through the update/reconcile path. When component filters caused the apparent child set to differ (e.g. filtered wrappers around stable children), this led to an invariant crash ("The children should not have changed if we pass in the same set.").

The change introduces a dedicated branch for prevWasHidden && !nextIsHidden that mounts the next child set and marks the subtree for child-order reset, mirroring the existing mount behavior used for other visibility transitions. A regression test covers the scenario with stable children from props and a filtered wrapper component.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk.
• The change is small and localized to the Offscreen/Activity reconciliation logic, aligns with the existing mount/unmount semantics for hidden subtrees, and includes a targeted regression test that previously triggered a deterministic crash.
• No files require special attention

Important Files Changed

Filename	Overview
packages/react-devtools-shared/src/backend/fiber/renderer.js	Fixes Offscreen (Activity) hidden→visible transition by mounting children instead of attempting to reconcile an unmounted child set; aligns with existing hidden/unmount behavior and prevents "children should not have changed" crash.
packages/react-devtools-shared/src/tests/store-test.js	Adds regression test ensuring a newly-visible with stable children and a filtered wrapper reconciles without crashing and yields expected Store tree.

[greptile-apps]

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[everettbu]

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