perf: Skip the pre-draw repair on frames without React mounts

[MatiPl01]

The pre-draw repair re-asserts every running platform transition on every frame, but React can only overwrite an animated value on frames where it wrote props. A UIManagerListener records mounts and the repair returns immediately on all other frames, so the walk over running transitions happens at write rate rather than at the display refresh rate.

That matters most after a transition settles: a persistent hold keeps its entry, so without the gate every later frame the window draws keeps walking the map. Measured on an emulator during a 3s transition, 201 of 240 pre-draw callbacks returned early.

Synchronous updates run their mount item inline (FabricUIManager.synchronouslyUpdateViewOnUIThread), bypassing MountItemDispatcher, so no listener fires for them even though opacity is one of the props they carry. They therefore open the gate explicitly, which makes the invariant true by construction rather than by coincidence. Measured on device, such writes are rare and always landed on a frame where a mount had already opened the gate, so this is insurance rather than an observed fix.

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• packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved animation value recovery after UI updates.
  • Prevented unnecessary visual repairs when no changes occurred since the previous frame, helping maintain smoother rendering.
  • Improved animation stability by applying corrections only when needed.
  • Preserved ongoing animations while reducing redundant rendering work and avoiding visual interruptions.
  • Improved cleanup during animation lifecycle changes to prevent stale UI update tracking.

Walkthrough

The CSS transitions manager tracks Fabric mount completion between draws. It skips clobbered-value repairs when no mount occurred, preserves animator state reporting, and unregisters the listener during cleanup.

Changes

CSS transition repair

Layer / File(s)	Summary
Mount tracking and conditional repair packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt	Registers a UIManagerListener with FabricUIManager. Mount completion sets mountedSinceLastDraw. repairClobberedValues skips repair when no mount occurred, preserves the animator-presence result, and resets the flag before repair. invalidate unregisters the listener.

Estimated code review effort: 2 (Simple) | ~10 minutes

Mergeability Score: ⚪ Minimal · up to 6957b

This localized performance change has no actionable merge-blocking risk remaining after normal checks and review.

Possibly related PRs

• software-mansion/react-native-reanimated#10090: Extends CSSPlatformTransitionsManager with related mount-tracking and repair logic.
• software-mansion/react-native-reanimated#10092: Modifies the related transition repair flow.
• software-mansion/react-native-reanimated#10170: Adds related mount-commit tracking in CSSPlatformTransitionsManager.kt.

Suggested reviewers: tjzel

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly summarizes the main change: skipping pre-draw repair on frames without React mounts.
Description check	✅ Passed	The description directly explains the UIManagerListener gate, early returns, synchronous updates, and the resulting performance improvement.

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• Commit unit tests in branch @matipl01/android-css-mount-gated-repair

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In
`@packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt`:
- Around line 32-48: Update CSSPlatformTransitionsManager to retain the
UIManagerListener registered in init, then remove that same listener during
teardown via removeUIManagerEventListener. Ensure NativeProxy.invalidate()
invokes the manager cleanup and clears its reference so the listener is detached
before a new CSSPlatformTransitionsManager is created.
- Around line 40-42: Update
CSSPlatformTransitionsManager.didMountItems(UIManager) so mountedSinceLastDraw
is set only when an actual React mount-item batch was dispatched, not for
command-only callbacks where the mount list is null. Track or consume the
preserved mount-item distinction through the relevant dispatch/listener flow,
while keeping command-only frames from triggering repairClobberedValues().

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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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• packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt

[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt (1)

77-84: 🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Handle zero-duration delayed transitions.

durationMs == 0 with a positive delay reaches animateTransition. In this case, delayFraction == 1f, so the final interpolation returns 0f and never writes toValue.

Add an explicit full-delay case and a regression test.

Proposed fix

-        override fun getInterpolation(input: Float): Float =
-            if (input <= delayFraction) 0f else inner.getInterpolation((input - delayFraction) / (1f - delayFraction))
+        override fun getInterpolation(input: Float): Float =
+            when {
+                delayFraction >= 1f -> if (input >= 1f) 1f else 0f
+                input <= delayFraction -> 0f
+                else -> inner.getInterpolation((input - delayFraction) / (1f - delayFraction))
+            }

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In
`@packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt`
around lines 77 - 84, Update HoldThenEase and the animateTransition path to
handle durationMs == 0 with a positive delay: ensure the delayed transition
reaches and writes toValue instead of remaining at 0f, while preserving normal
interpolation for nonzero durations. Add a regression test covering a
zero-duration delayed transition and verifying the final value is applied.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Outside diff comments:
In
`@packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt`:
- Around line 77-84: Update HoldThenEase and the animateTransition path to
handle durationMs == 0 with a positive delay: ensure the delayed transition
reaches and writes toValue instead of remaining at 0f, while preserving normal
interpolation for nonzero durations. Add a regression test covering a
zero-duration delayed transition and verifying the final value is applied.

---

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📒 Files selected for processing (1)

• packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt