[iOS][Fabric] Dynamic borderColor/outlineColor (PlatformColor/DynamicColorIOS) resolve against the system appearance, ignoring overrideUserInterfaceStyle

Reproducer

Single-file RNTester reproducer — edits only RNTesterPlayground.js:

• Branch: https://github.com/JacquesLeupin/react-native/tree/repro/fabric-border-trait-collection-57836
• Diff vs main: https://github.com/facebook/react-native/compare/main...JacquesLeupin:react-native:repro/fabric-border-trait-collection-57836

Run RNTester on iOS with the simulator's System Appearance set to Light and open the Playground example: it forces the app dark via Appearance.setColorScheme('dark') at 2s and mounts a second row of boxes at 4s (a button resets to the system scheme and re-runs). The same reproducer as a standalone stock-CLI app is inline under Steps to reproduce below.

Description

On the New Architecture, borderColor (all edges) and outlineColor set from a dynamic color (PlatformColor or DynamicColorIOS) resolve against the system appearance instead of the view's effective appearance. When an app forces an appearance that differs from the system — Appearance.setColorScheme('dark') (which sets overrideUserInterfaceStyle on every window) or a native overrideUserInterfaceStyle assignment — borders render the wrong appearance variant while backgroundColor and text render correctly.

Worse, the wrong border persists. Two shapes (both observed in the reproducer / a production app):

• Views already mounted when the override flips keep their stale borders: traitCollectionDidChange: → invalidateLayer exists for exactly this, but the border conversion inside it still reads the ambient trait state rather than the view's own traits, and the borders demonstrably stay on the pre-override variant while backgroundColor (a UIView-managed dynamic color) corrects itself.
• Recycled component views reattach under unchanged traits, so no trait-change callback ever fires for them; whatever invalidateLayer resolved during a mounting pass (ambient = system appearance) sticks. In a production app with list recycling this makes bordered surfaces render system-variant borders indefinitely under a launch-time override.

The old architecture renders this correctly: Paper resolves border colors against the view's trait collection in RCTView (displayLayer: via borderColorsWithTraitCollection:), so this is a Fabric regression relative to Paper.

Root cause

In React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm, invalidateLayer resolves backgroundColor explicitly against the view's trait collection:

UIColor *backgroundColor = [_backgroundColor resolvedColorWithTraitCollection:self.traitCollection];

…but the border and outline paths convert dynamic UIColors straight to CGColor with no explicit resolve:

UIColor *borderColor = RCTUIColorFromSharedColor(borderMetrics.borderColors.left);
layer.borderColor = borderColor.CGColor;   // resolves via UITraitCollection.currentTraitCollection

-[UIColor CGColor] on a dynamic color resolves against UITraitCollection.currentTraitCollection, which tracks the system appearance — Fabric mounting runs outside UIKit's trait-context callbacks, so it never matches a window-level overrideUserInterfaceStyle. The same flattening affects RCTCreateRCTBorderColorsFromBorderColors (the border-image path) and both outlineColor paths. (layer.shadowColor in updateProps has the same latent issue.)

Steps to reproduce

1. Set the iOS Simulator/device System Appearance to Light.
2. Create a stock app (npx @react-native-community/cli init — New Architecture default) and use the App.tsx below.
3. Launch. The app forces dark via Appearance.setColorScheme('dark') after 2s, then mounts a second pair of boxes at 4s.

import React, {useEffect, useState} from 'react';
import {Appearance, DynamicColorIOS, StyleSheet, Text, useColorScheme, View} from 'react-native';

// ONE dynamic color for both fills and borders: red in light, green in dark.
const dynamicColor = DynamicColorIOS({light: '#ff3b30', dark: '#34c759'});

function Boxes({label}: {label: string}) {
  return (
    <View style={styles.section}>
      <Text style={styles.sectionLabel}>{label}</Text>
      <View style={styles.row}>
        <View style={[styles.box, {backgroundColor: dynamicColor}]} />
        <View style={[styles.box, styles.bordered, {borderColor: dynamicColor}]} />
        <View style={[styles.box, styles.bordered, styles.clipped, {borderColor: dynamicColor}]} />
      </View>
    </View>
  );
}

export default function App() {
  const scheme = useColorScheme();
  const [afterOverride, setAfterOverride] = useState(false);
  useEffect(() => {
    const t1 = setTimeout(() => Appearance.setColorScheme('dark'), 2000);
    const t2 = setTimeout(() => setAfterOverride(true), 4000);
    return () => { clearTimeout(t1); clearTimeout(t2); };
  }, []);
  return (
    <View style={styles.screen}>
      <Text style={styles.title}>useColorScheme(): {String(scheme)}</Text>
      <Boxes label="A: mounted BEFORE the dark override" />
      {afterOverride && <Boxes label="B: mounted AFTER the dark override" />}
    </View>
  );
}

const styles = StyleSheet.create({
  screen: {flex: 1, paddingTop: 90, paddingHorizontal: 24, backgroundColor: '#202124'},
  title: {fontSize: 18, fontWeight: '600', color: '#fff'},
  section: {marginTop: 16},
  sectionLabel: {fontSize: 14, color: '#fff', marginBottom: 8},
  row: {flexDirection: 'row', gap: 12},
  clipped: {overflow: 'hidden'},
  box: {width: 104, height: 80, borderRadius: 8},
  bordered: {borderWidth: 6},
});

The three boxes per row exercise backgroundColor, the border-image path (default overflow), and the CoreAnimation layer.borderColor path (overflow: 'hidden').

Expected: once the app is dark (useColorScheme() reports dark), every fill and border is green.

Actual: every fill is green, but section A's borders — on both border code paths — remain red (the light variant) permanently after the override flips. Freshly-created views in section B can render correctly via the attach-time trait change; recycled views (the common case in real list-heavy apps) reattach with unchanged traits and stay wrong, which is how this presents across whole production screens.

Version and platforms

• Reproduced on 0.81.5 and 0.86.2 (New Architecture); the affected code is unchanged on main (f7a8360696).
• iOS only. Old architecture renders correctly.

Related

• #42006 (closed) — PlatformColor/DynamicColorIOS regressions on the New Architecture; the "only the initial scheme applies" half of that report matches this mechanism for borders.
• #30377 (closed, 2020) — the Paper-era version of dynamic border colors not updating, fixed for Paper via trait-collection resolution in RCTView.

A fix PR that mirrors the existing backgroundColor handling (resolve against self.traitCollection at all four border/outline conversion sites) accompanies this issue.

Start in packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm and inspect invalidateLayer plus the border-image and outline conversion paths. Run the RNTester Playground reproducer on iOS with a light system appearance and a forced dark override. Done means dynamic border and outline colors follow the view's effective trait collection for mounted, newly mounted, and recycled views, while background colors remain correct.