fix: transition triggered from default values when config is attached to a mounted view

[MatiPl01]

Adding a transition config to an already-mounted view started a transition for every styled property from its default value, so a view resting at opacity: 0.2 flashed to full opacity and animated back down.

While no config was attached, the manager kept the caller's empty style snapshot as its diffing baseline, so the first configured update diffed the real style against {}. The baseline is now only ever a real snapshot, which makes attaching behave like mounting: it arms silently, and only later changes animate.

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• packages/react-native-reanimated/src/css/native/managers/__tests__/CSSManager.test.ts

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Fixed CSS transitions occasionally comparing changes against an artificial baseline.
  • Prevented unintended animations during animation-only updates.
  • Ensured transitions attached to unchanged values do not animate.
  • Improved handling when no normalized style is available.
  • Improved consistency of subsequent transitions after style updates.

• Tests

  • Added coverage for transition baselines, empty transition configurations, unchanged style values, and animation-only updates.

Walkthrough

CSS transition updates now distinguish missing style snapshots from empty styles. CSS manager wiring preserves optional normalized styles, and tests cover transition baselines across animation-only, normalized-empty, and unchanged-value updates.

Changes

CSS transition baseline preservation

Layer / File(s)	Summary
Preserve style transition baselines packages/react-native-reanimated/src/css/native/managers/CSSManager.ts, packages/react-native-reanimated/src/css/native/managers/CSSTransitionsManager.ts, packages/react-native-reanimated/src/css/native/managers/__tests__/CSSManager.test.ts	CSSManager passes normalizedStyle without an empty-object fallback. CSSTransitionsManager stores null when no style snapshot exists. Tests validate baseline preservation and unchanged values.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 8c617

The change prevents transitions from starting at default property values when configuration is attached to an already-mounted view; no actionable merge-blocking risk remains after normal checks and review.

Possibly related PRs

• software-mansion/react-native-reanimated#10119: Both PRs modify CSSTransitionsManager transition handling and runCSSTransition invocation.
• software-mansion/react-native-reanimated#10124: Both PRs modify CSSTransitionsManager.update and CSSManager transition-update wiring.
• software-mansion/react-native-reanimated#10252: Both PRs modify CSSManager.update and related CSS transition handling.

Suggested reviewers: tjzel, pawicao

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the transition-baseline bug and the fix for mounted views.
Title check	✅ Passed	The title clearly identifies the fix for transitions triggered by default values when configuration is attached to a mounted view.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch @MatiPl01/fix-transition-attach-baseline

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Actionable comments posted: 1

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Inline comments:
In
`@packages/react-native-reanimated/src/css/native/managers/__tests__/CSSManager.test.ts`:
- Around line 216-223: Extend the test around the manager update sequence to
perform a subsequent update to opacity 1 after attaching TRANSITION, then assert
transitionedValues() equals [[0.2, 1]]. Preserve the existing assertion that
attaching the transition config itself produces no transition.

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Fix all unresolved CodeRabbit comments on this PR:

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• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 090e8888-0a31-43bc-8b54-55d77e9a811a

📥 Commits

Reviewing files that changed from the base of the PR and between 4e82ad4 and 2e8ac68.

📒 Files selected for processing (1)

• packages/react-native-reanimated/src/css/native/managers/__tests__/CSSManager.test.ts

Included review availability: Your plan includes up to 10 reviews per rolling hour; 8 remain after this review.