Optimize flattenStyle for nested style arrays

[tarikfp]

Summary

• Walk nested style arrays directly into one result object instead of recursively allocating intermediate flattened objects.
• Preserve existing behavior for object styles, falsy entries, and later-style override order.
• Add focused unit coverage for array inputs that must still allocate merged result objects.

Benchmark proof

External reproducible benchmark app:
https://github.com/tarikfp/rn-style-flatten-benchmark

Latest yarn bench:compare from the benchmark repo compares React Native main at 066c0d8bd8 against this branch at 81b5bc26b6:

scenario	before median ms	after median ms	change
nested single style array	294.79	98.73	66.5% faster
nested merged style array	278.54	122.06	56.2% faster

This is not a blanket claim that every React Native screen becomes 50%+ faster. It is targeted to the flattenStyle path when composed components pass nested style arrays.

Validation

• yarn test in benchmark app repo
• yarn lint in benchmark app repo
• yarn workspace style-flatten-benchmark-app tsc --noEmit
• yarn bench:compare
• yarn test packages/react-native/Libraries/StyleSheet/__tests__/flattenStyle-test.js --runInBand in the React Native checkout
• iOS simulator build/install for the benchmark app

Draft while broader upstream validation is gathered.

Affected areas

flattenStyle is not only the public StyleSheet.flatten helper. Core React Native components call it when they need to read or normalize style props before passing work further down:

• Text uses it in packages/react-native/Libraries/Text/Text.js:188 before normalizing text style values such as numeric fontWeight and text selection-related props.
• Image uses it on both platforms: Image.ios.js:141 reads objectFit, resizeMode, and tintColor; Image.android.js:310 reads objectFit and resizeMode before building native props.
• ImageBackground uses it in packages/react-native/Libraries/Image/ImageBackground.js:72 before splitting size-related style values between the wrapper view and inner image.
• TextInput uses it in packages/react-native/Libraries/Components/TextInput/TextInput.js:655 while preserving the original style when possible, but still flattening to normalize text style overrides.
• TouchableOpacity uses it in packages/react-native/Libraries/Components/Touchable/TouchableOpacity.js:260 and :364 to read opacity from style when setting and updating its animated opacity.
• ScrollView uses it in packages/react-native/Libraries/Components/ScrollView/ScrollView.js:1663 for development-time layout warnings and in :1850 when splitting outer and inner layout props around refresh controls.
• Animated.ScrollView uses the same split path in packages/react-native/Libraries/Animated/components/AnimatedScrollView.js:94.
• Animated props use it in packages/react-native/Libraries/Animated/nodes/AnimatedProps.js:62 and :157, and the newer memo hook uses it in packages/react-native/src/private/animated/createAnimatedPropsMemoHook.js:125, so nested style arrays also matter for animated style props.
• Fabric public instances use it in packages/react-native/Libraries/ReactNative/ReactFabricPublicInstance/ReactNativeAttributePayload.js:186 and :195 before diffing array style props.
• Developer tooling also goes through it: the element inspector uses it in ElementProperties.js:43 and ElementBox.js:34, and React DevTools receives it as the React Native style resolver from setUpReactDevTools.js:74.

That is why the benchmark focuses on nested style arrays instead of claiming every render gets faster. The change helps when those call sites receive styles composed like style={[base, condition && extra, [override]]}.

Changelog:

[GENERAL] [CHANGED] - Make flattenStyle avoid extra intermediate objects when flattening nested style arrays.

Test Plan:

• Ran the focused React Native unit test: yarn test packages/react-native/Libraries/StyleSheet/__tests__/flattenStyle-test.js --runInBand.
• Ran the external benchmark app checks: yarn test, yarn lint, and yarn workspace style-flatten-benchmark-app tsc --noEmit.
• Ran yarn bench:compare in the benchmark app repo. The latest saved result compares React Native main at 066c0d8bd8 with this branch at 81b5bc26b6 and shows the nested style-array cases improving from 294.79 ms to 98.73 ms and from 278.54 ms to 122.06 ms.
• Built and installed the benchmark app on iOS simulators to compare a main build and this branch side by side.

[meta-codesync]

@Abbondanzo has imported this pull request. If you are a Meta employee, you can view this in D108616011.