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fix(ios): prevent missing hairlineWidth border strokes at subpixel Y … #58162

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fix(ios): prevent missing hairlineWidth border strokes at subpixel Y … #58162

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5be76bc
fix(ios): prevent missing hairlineWidth border strokes at subpixel Y …
shashikant-panchal Aug 27, 2026
d28e05c
test(rn-tester): add hairline border test at subpixel Y offsets
shashikant-panchal Sep 10, 2026
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4 changes: 3 additions & 1 deletion packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm
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Original file line number Diff line number Diff line change
Expand Up @@ -887,7 +887,9 @@ static void RCTAddContourEffectToLayer(
UIEdgeInsets imageCapInsets = image.capInsets;
CGRect contentsCenter = CGRect{
CGPoint{imageCapInsets.left / imageSize.width, imageCapInsets.top / imageSize.height},
CGSize{(CGFloat)1.0 / imageSize.width, (CGFloat)1.0 / imageSize.height}};
CGSize{
MAX(0, (imageSize.width - imageCapInsets.left - imageCapInsets.right) / imageSize.width),
MAX(0, (imageSize.height - imageCapInsets.top - imageCapInsets.bottom) / imageSize.height)}};
layer.contents = (id)image.CGImage;
layer.contentsScale = image.scale;

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8 changes: 2 additions & 6 deletions packages/react-native/React/Views/RCTBorderDrawing.m
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Original file line number Diff line number Diff line change
Expand Up @@ -247,12 +247,8 @@ static CGPathRef RCTPathCreateOuterOutline(BOOL drawToEdge, CGRect rect, RCTCorn

const CGSize size = makeStretchable ? (CGSize){
// 1pt for the middle stretchable area along each axis
// we also need to round the edge insets to avoid border bleeding
// this is because if the size is decimal, when calculating the unit
// rectangle for CALayer.contentsCenter we encounter rounding errors
// which causes visual glitches
ceil(edgeInsets.left) + 1 + ceil(edgeInsets.right),
ceil(edgeInsets.top) + 1 + ceil(edgeInsets.bottom),
edgeInsets.left + 1 + edgeInsets.right,
edgeInsets.top + 1 + edgeInsets.bottom,
} : viewSize;

UIGraphicsImageRenderer *const imageRenderer =
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5 changes: 4 additions & 1 deletion packages/react-native/React/Views/RCTView.m
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Original file line number Diff line number Diff line change
Expand Up @@ -860,7 +860,10 @@ - (void)displayLayer:(CALayer *)layer
CGSize size = image.size;
UIEdgeInsets insets = image.capInsets;
CGRectMake(
insets.left / size.width, insets.top / size.height, (CGFloat)1.0 / size.width, (CGFloat)1.0 / size.height);
insets.left / size.width,
insets.top / size.height,
MAX(0, (size.width - insets.left - insets.right) / size.width),
MAX(0, (size.height - insets.top - insets.bottom) / size.height));
});

layer.contents = (id)image.CGImage;
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131 changes: 131 additions & 0 deletions packages/rn-tester/js/examples/Border/BorderExample.js
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@
import type {RNTesterModule} from '../../types/RNTesterTypes';

import hotdog from '../../assets/hotdog.jpg';
import RNTesterText from '../../components/RNTesterText';
import * as React from 'react';
import {
DynamicColorIOS,
Expand Down Expand Up @@ -257,6 +258,33 @@ const styles = StyleSheet.create({
top: -10,
backgroundColor: 'blue',
},
hairlineBox: {
width: 44,
height: 44,
borderWidth: StyleSheet.hairlineWidth,
borderColor: 'black',
backgroundColor: '#f5f5f5',
},
hairlineBoxContainer: {
alignItems: 'center',
marginRight: 8,
},
hairlineRow: {
borderBottomWidth: StyleSheet.hairlineWidth,
borderColor: 'black',
justifyContent: 'center',
paddingHorizontal: 4,
},
subpixelLabel: {
fontSize: 9,
marginTop: 4,
},
sectionTitle: {
fontSize: 11,
fontWeight: 'bold',
marginBottom: 6,
marginTop: 10,
},
});

export default {
Expand Down Expand Up @@ -622,5 +650,108 @@ export default {
);
},
},
{
title: 'Hairline borders at subpixel Y offsets',
name: 'hairline-subpixel-y-offsets',
description:
'Ensure hairlineWidth borders render completely on all sides even when positioned at fractional/subpixel Y offsets on iOS',
render: function (): React.Node {
const subpixelYOffsets = [0, 0.25, 0.33, 0.5, 0.67, 0.75];
return (
<View testID="border-test-hairline-subpixel-y-offsets">
<RNTesterText style={styles.sectionTitle}>
Boxes at fractional Y offsets (borderWidth = hairlineWidth):
</RNTesterText>
<View style={styles.wrapper}>
{subpixelYOffsets.map(yOffset => (
<View key={yOffset} style={styles.hairlineBoxContainer}>
<View style={[styles.hairlineBox, {top: yOffset}]} />
<RNTesterText style={styles.subpixelLabel}>
{`+${yOffset}pt`}
</RNTesterText>
</View>
))}
</View>

<RNTesterText style={styles.sectionTitle}>
Stacked rows with hairline bottom borders (fractional heights):
</RNTesterText>
<View>
{subpixelYOffsets.map((fraction, index) => (
<View
key={index}
style={[styles.hairlineRow, {height: 24 + fraction}]}>
<RNTesterText style={styles.subpixelLabel}>
{`Row ${index + 1} (height: ${(24 + fraction).toFixed(2)}pt)`}
</RNTesterText>
</View>
))}
</View>

<RNTesterText style={styles.sectionTitle}>
Single-side hairline borders at fractional Y offset (+0.33pt):
</RNTesterText>
<View style={styles.wrapper}>
<View style={styles.hairlineBoxContainer}>
<View
style={[
styles.hairlineBox,
{
borderWidth: 0,
borderTopWidth: StyleSheet.hairlineWidth,
borderColor: 'red',
top: 0.33,
},
]}
/>
<RNTesterText style={styles.subpixelLabel}>top</RNTesterText>
</View>
<View style={styles.hairlineBoxContainer}>
<View
style={[
styles.hairlineBox,
{
borderWidth: 0,
borderBottomWidth: StyleSheet.hairlineWidth,
borderColor: 'red',
top: 0.33,
},
]}
/>
<RNTesterText style={styles.subpixelLabel}>bottom</RNTesterText>
</View>
<View style={styles.hairlineBoxContainer}>
<View
style={[
styles.hairlineBox,
{
borderWidth: 0,
borderLeftWidth: StyleSheet.hairlineWidth,
borderColor: 'red',
top: 0.33,
},
]}
/>
<RNTesterText style={styles.subpixelLabel}>left</RNTesterText>
</View>
<View style={styles.hairlineBoxContainer}>
<View
style={[
styles.hairlineBox,
{
borderWidth: 0,
borderRightWidth: StyleSheet.hairlineWidth,
borderColor: 'red',
top: 0.33,
},
]}
/>
<RNTesterText style={styles.subpixelLabel}>right</RNTesterText>
</View>
</View>
</View>
);
},
},
],
} as RNTesterModule;