[Android] Align the alpha traversal gate with iOS

[m-bert]

Description

The orchestrator skips views with alpha below minimumAlphaForTraversal while looking for handlers. RNGestureHandlerRootHelper has set it to 0.1 since 2017, while iOS (UIKit and RCTViewComponentView) stops hit testing at 0.01, and React Native's own Android responder does not check alpha at all. Any RNGH handler under a view with opacity between 0.01 and 0.1 was unreachable on Android only.

Two changes:

1. MIN_ALPHA_FOR_TOUCH goes from 0.1 to 0.01, matching the iOS threshold.

2. The box-none shortcut for RNGestureHandlerDetectorView recorded the detector's own handlers whenever no child consumed the touch, so that hitSlop keeps working. It did not distinguish a bounds miss from a child skipped for being hidden or below the alpha gate, so a v3 GestureDetector with a fully transparent child stayed tappable. The shortcut now requires at least one child that can receive events.

Test plan

Opacity hit-test matrix screen below (GD Tap / RectButton / GH Pressable / RN Pressable at opacities 1 .. 0, on the target, on a wrapper and as transparent overlays), run with argent on a Pixel 9 Pro emulator, both v3 and legacy engines.

Before: RNGH targets fired down to 0.1 and were dead at 0.09 and below. A v3 GestureDetector with the opacity on its child fired at every opacity including 0.

After: all RNGH targets fire down to 0.01 and are dead at 0.005 and 0, on the target and on a wrapper, including the v3 GestureDetector. Overlays at 0.05 now block RNGH targets the same way they do on iOS; opacity-0 overlays are still skipped. RN Pressable is unchanged and still fires at every opacity, since RN's responder has no alpha gate.

Tested on the following code:

import React, { useCallback, useMemo, useState } from 'react';
import {
  Platform,
  Pressable as RNPressable,
  ScrollView,
  StyleSheet,
  Text,
  View,
} from 'react-native';
import {
  Gesture,
  GestureDetector,
  LegacyPressable,
  LegacyRectButton,
  Pressable,
  RectButton,
  useTapGesture,
} from 'react-native-gesture-handler';

// Opacity hit-testing matrix (roadmap: "Investigate minimalAlphaTraversal on
// Android"). Every cell is a tappable target rendered at a given opacity; the
// number under it counts fires. Known gates:
//  - Android RNGH orchestrator: skips views with alpha < 0.1 while looking for
//    handlers (minimumAlphaForTraversal, RNGestureHandlerRootHelper).
//  - Android RN responder / native dispatch: no alpha check at all.
//  - iOS UIKit hitTest (and RN's RCTViewComponentView): skips alpha < 0.01,
//    but RNGestureHandlerButtonComponentView.hitTest forwards straight to the
//    button and skips that check.
//  - Web: DOM hit testing ignores opacity entirely.
// Sections:
//  1. opacity on the target view itself
//  2. opacity on a plain wrapper View above the target
//  3. transparent overlay (plain View / RN Pressable / RectButton) on top
//     of an opaque target - does it swallow the tap? (#3223 shape)
// The engine toggle switches every RNGH cell between the v3 components/hooks
// and the legacy ones; counts are kept per engine.

const OPACITIES = [1, 0.5, 0.1, 0.09, 0.05, 0.01, 0.005, 0];
const OVERLAY_OPACITIES = [0.05, 0];

type Engine = 'v3' | 'legacy';

type Kind = 'tap' | 'button' | 'pressable' | 'rn';
const KINDS: Kind[] = ['tap', 'button', 'pressable', 'rn'];
const KIND_LABEL: Record<Kind, string> = {
  tap: 'GD Tap',
  button: 'RectButton',
  pressable: 'GH Pressable',
  rn: 'RN Pressable',
};

type OverlayKind = 'view' | 'rn' | 'button';
const OVERLAY_KINDS: OverlayKind[] = ['view', 'rn', 'button'];
const OVERLAY_LABEL: Record<OverlayKind, string> = {
  view: 'plain View',
  rn: 'RN Pressable',
  button: 'RectButton',
};

type Bump = (key: string) => void;

const Target = React.memo(function Target({
  engine,
  kind,
  fireKey,
  bump,
  style,
}: {
  engine: Engine;
  kind: Kind;
  fireKey: string;
  bump: Bump;
  style?: object;
}) {
  const onFire = useCallback(() => bump(fireKey), [bump, fireKey]);
  const legacyTap = useMemo(
    () => Gesture.Tap().runOnJS(true).onStart(onFire),
    [onFire]
  );
  const v3Tap = useTapGesture({
    onActivate: onFire,
    runOnJS: true,
  });
  const targetStyle = [styles.target, style];

  switch (kind) {
    case 'tap':
      return engine === 'v3' ? (
        <GestureDetector gesture={v3Tap}>
          <View style={targetStyle} />
        </GestureDetector>
      ) : (
        <GestureDetector gesture={legacyTap}>
          <View style={targetStyle} />
        </GestureDetector>
      );
    case 'button':
      return engine === 'v3' ? (
        <RectButton
          style={targetStyle}
          rippleColor="#ffffff"
          onPress={onFire}
        />
      ) : (
        <LegacyRectButton
          style={targetStyle}
          rippleColor="#ffffff"
          onPress={onFire}
        />
      );
    case 'pressable':
      return engine === 'v3' ? (
        <Pressable style={targetStyle} onPress={onFire} />
      ) : (
        <LegacyPressable style={targetStyle} onPress={onFire} />
      );
    case 'rn':
      return <RNPressable style={targetStyle} onPress={onFire} />;
  }
});

const Overlay = React.memo(function Overlay({
  engine,
  kind,
  opacity,
  fireKey,
  bump,
}: {
  engine: Engine;
  kind: OverlayKind;
  opacity: number;
  fireKey: string;
  bump: Bump;
}) {
  const onFire = useCallback(() => bump(fireKey), [bump, fireKey]);
  const style = [StyleSheet.absoluteFill, styles.overlay, { opacity }];
  switch (kind) {
    case 'view':
      return <View style={style} />;
    case 'rn':
      return <RNPressable style={style} onPress={onFire} />;
    case 'button':
      return engine === 'v3' ? (
        <RectButton style={style} onPress={onFire} />
      ) : (
        <LegacyRectButton style={style} onPress={onFire} />
      );
  }
});

const Cell = React.memo(function Cell({
  count,
  overlayCount,
  children,
}: {
  count: number;
  overlayCount?: number;
  children: React.ReactNode;
}) {
  const fired = count > 0;
  const swallowed = !fired && (overlayCount ?? 0) > 0;
  return (
    <View
      style={[
        styles.cell,
        fired && styles.cellFired,
        swallowed && styles.cellSwallowed,
      ]}>
      <View style={styles.targetArea}>{children}</View>
      <Text style={styles.count}>
        {overlayCount === undefined ? count : `${count} / ov ${overlayCount}`}
      </Text>
    </View>
  );
});

function fmt(n: number) {
  return String(n);
}

export default function EmptyExample() {
  const [engine, setEngine] = useState<Engine>('v3');
  const [counts, setCounts] = useState<Record<string, number>>({});
  const bump = useCallback<Bump>((key) => {
    setCounts((prev) => ({ ...prev, [key]: (prev[key] ?? 0) + 1 }));
  }, []);
  const reset = useCallback(() => setCounts({}), []);
  const toggleEngine = useCallback(
    () => setEngine((prev) => (prev === 'v3' ? 'legacy' : 'v3')),
    []
  );
  const get = (key: string) => counts[key] ?? 0;

  const summary = useMemo(() => {
    const lines: string[] = [];
    for (const eng of ['v3', 'legacy'] as const) {
      for (const section of ['target', 'wrapper'] as const) {
        for (const kind of KINDS) {
          const fired = OPACITIES.filter(
            (o) => (counts[`${eng}|${section}|${o}|${kind}`] ?? 0) > 0
          );
          if (fired.length > 0) {
            lines.push(
              `${eng} ${section}/${KIND_LABEL[kind]}: ${fired.join(', ')}`
            );
          }
        }
      }
      for (const ok of OVERLAY_KINDS) {
        for (const oo of OVERLAY_OPACITIES) {
          const reached = KINDS.filter(
            (k) => (counts[`${eng}|overlay|${ok}|${oo}|${k}`] ?? 0) > 0
          );
          const swallowed = KINDS.filter(
            (k) =>
              (counts[`${eng}|overlay|${ok}|${oo}|${k}`] ?? 0) === 0 &&
              (counts[`${eng}|overlay-ov|${ok}|${oo}|${k}`] ?? 0) > 0
          );
          if (reached.length > 0 || swallowed.length > 0) {
            lines.push(
              `${eng} overlay ${OVERLAY_LABEL[ok]}@${oo}: reached ${
                reached.map((k) => KIND_LABEL[k]).join(', ') || '-'
              }; swallowed ${
                swallowed.map((k) => KIND_LABEL[k]).join(', ') || '-'
              }`
            );
          }
        }
      }
    }
    return lines;
  }, [counts]);

  const header = (
    <View style={styles.row}>
      <Text style={[styles.rowLabel, styles.headerText]}>opacity</Text>
      {KINDS.map((k) => (
        <Text key={k} style={[styles.colHeader, styles.headerText]}>
          {KIND_LABEL[k]}
        </Text>
      ))}
    </View>
  );

  return (
    <ScrollView contentContainerStyle={styles.container}>
      <Text style={styles.title}>
        Opacity hit testing ({Platform.OS}, {engine})
      </Text>
      <Text style={styles.note}>
        Tap every cell once. Green = target fired, red = only the overlay fired.
        Gates: Android RNGH 0.1, iOS 0.01, RN Android & web none. Summary is at
        the bottom.
      </Text>
      <View style={styles.row}>
        <RectButton style={styles.resetButton} onPress={reset}>
          <Text style={styles.resetText}>Reset</Text>
        </RectButton>
        <RectButton style={styles.resetButton} onPress={toggleEngine}>
          <Text style={styles.resetText}>Engine: {engine}</Text>
        </RectButton>
      </View>

      <Text style={styles.section}>1. Opacity on the target view</Text>
      {header}
      {OPACITIES.map((o) => (
        <View key={o} style={styles.row}>
          <Text style={styles.rowLabel}>{fmt(o)}</Text>
          {KINDS.map((kind) => {
            const key = `${engine}|target|${o}|${kind}`;
            return (
              <Cell key={kind} count={get(key)}>
                <Target
                  engine={engine}
                  kind={kind}
                  fireKey={key}
                  bump={bump}
                  style={{ opacity: o }}
                />
              </Cell>
            );
          })}
        </View>
      ))}

      <Text style={styles.section}>2. Opacity on a wrapper View</Text>
      {header}
      {OPACITIES.map((o) => (
        <View key={o} style={styles.row}>
          <Text style={styles.rowLabel}>{fmt(o)}</Text>
          {KINDS.map((kind) => {
            const key = `${engine}|wrapper|${o}|${kind}`;
            return (
              <Cell key={kind} count={get(key)}>
                <View style={[styles.fill, { opacity: o }]}>
                  <Target
                    engine={engine}
                    kind={kind}
                    fireKey={key}
                    bump={bump}
                  />
                </View>
              </Cell>
            );
          })}
        </View>
      ))}

      <Text style={styles.section}>
        3. Transparent overlay above an opaque target
      </Text>
      {header}
      {OVERLAY_KINDS.map((ok) =>
        OVERLAY_OPACITIES.map((oo) => (
          <View key={`${ok}-${oo}`} style={styles.row}>
            <Text style={styles.rowLabel}>
              {OVERLAY_LABEL[ok]}
              {'\n'}@ {fmt(oo)}
            </Text>
            {KINDS.map((kind) => {
              const key = `${engine}|overlay|${ok}|${oo}|${kind}`;
              const ovKey = `${engine}|overlay-ov|${ok}|${oo}|${kind}`;
              return (
                <Cell key={kind} count={get(key)} overlayCount={get(ovKey)}>
                  <Target
                    engine={engine}
                    kind={kind}
                    fireKey={key}
                    bump={bump}
                  />
                  <Overlay
                    engine={engine}
                    kind={ok}
                    opacity={oo}
                    fireKey={ovKey}
                    bump={bump}
                  />
                </Cell>
              );
            })}
          </View>
        ))
      )}

      <View style={styles.summary}>
        <Text style={styles.headerText}>Summary</Text>
        {summary.length === 0 ? (
          <Text style={styles.summaryLine}>nothing fired yet</Text>
        ) : (
          summary.map((line) => (
            <Text key={line} style={styles.summaryLine}>
              {line}
            </Text>
          ))
        )}
      </View>
    </ScrollView>
  );
}

const styles = StyleSheet.create({
  container: {
    padding: 8,
    paddingBottom: 48,
  },
  title: {
    fontSize: 18,
    fontWeight: '600',
    marginBottom: 4,
  },
  note: {
    fontSize: 12,
    color: '#555',
    marginBottom: 8,
  },
  resetButton: {
    backgroundColor: '#333',
    paddingHorizontal: 16,
    paddingVertical: 8,
    borderRadius: 6,
    marginBottom: 8,
    marginRight: 8,
  },
  resetText: {
    color: 'white',
    fontWeight: '600',
  },
  summary: {
    borderWidth: 1,
    borderColor: '#ccc',
    borderRadius: 6,
    padding: 8,
    marginTop: 12,
  },
  summaryLine: {
    fontSize: 11,
    fontVariant: ['tabular-nums'],
  },
  section: {
    fontSize: 15,
    fontWeight: '600',
    marginTop: 16,
    marginBottom: 4,
  },
  headerText: {
    fontSize: 11,
    fontWeight: '600',
  },
  row: {
    flexDirection: 'row',
    alignItems: 'center',
    marginBottom: 4,
  },
  rowLabel: {
    width: 64,
    fontSize: 11,
    fontVariant: ['tabular-nums'],
  },
  colHeader: {
    flex: 1,
    textAlign: 'center',
  },
  cell: {
    flex: 1,
    marginHorizontal: 2,
    borderWidth: 1,
    borderColor: '#bbb',
    borderRadius: 4,
    overflow: 'hidden',
  },
  cellFired: {
    borderColor: '#2e8b57',
    backgroundColor: '#e6f4ea',
  },
  cellSwallowed: {
    borderColor: '#c0392b',
    backgroundColor: '#fbe9e7',
  },
  targetArea: {
    height: 40,
  },
  fill: {
    flex: 1,
  },
  target: {
    flex: 1,
    backgroundColor: '#4a90d9',
  },
  overlay: {
    backgroundColor: '#e74c3c',
  },
  count: {
    fontSize: 10,
    textAlign: 'center',
    paddingVertical: 2,
    fontVariant: ['tabular-nums'],
  },
});

[coderabbitai]

📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved touch handling for gesture detectors whose child views are hidden or too transparent to receive events.
  • Preserved detector hit-area behavior when at least one child remains interactable.
  • Updated transparency handling so views with very low opacity respond to touches consistently with iOS behavior.

Walkthrough

Changes

Android touch handling

Layer / File(s)	Summary
Align minimum touch alpha threshold packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/react/RNGestureHandlerRootHelper.kt	MIN_ALPHA_FOR_TOUCH changes from 0.1f to 0.01f. A comment records alignment with iOS hit testing.
Filter hidden detector children packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/GestureHandlerOrchestrator.kt	The orchestrator detects view groups whose children cannot receive events. Detector self-handlers are recorded without child consumption only when an event-receivable child exists.

Suggested reviewers: j-piasecki

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to fc3cd

A supported detector wrapper can still respond to touches when its hidden nested targets should be unreachable; the correction is localized.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 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 and concisely describes the main change: aligning Android alpha traversal behavior with iOS.

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Pull request overview

Aligns Android gesture traversal with iOS alpha behavior and prevents transparent detector children from remaining tappable.

Changes:

• Lowers Android’s traversal alpha threshold from 0.1 to 0.01.
• Adds a child-visibility/alpha guard for detector handlers.

File summaries

File	Description
packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/react/RNGestureHandlerRootHelper.kt	Updated as part of this pull request.
packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/GestureHandlerOrchestrator.kt	Updated as part of this pull request.

Review details

Suppressed comments (2)

packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/GestureHandlerOrchestrator.kt:918

• This predicate only checks visibility and alpha, so a direct child with pointerEvents="none" is still treated as available. extractGestureHandlers skips that child as PointerEventsConfig.NONE, then this branch records the detector's handlers anyway, so a GestureDetector around a non-hit-testable child can still fire (including through its hitSlop). Include the child's pointer-events configuration in this check, at least excluding NONE, before using the shortcut.

  private fun allChildrenHidden(viewGroup: ViewGroup) =
    viewGroup.childCount > 0 && (0 until viewGroup.childCount).none { canReceiveEvents(viewGroup.getChildAt(it)) }

packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/GestureHandlerOrchestrator.kt:874

• The guard only checks alpha/visibility of direct children, not whether the child under this pointer can actually receive the event. With multiple detector children, a visible sibling makes allChildrenHidden false; a tap over a different child whose alpha is below the gate makes extractGestureHandlers return false, and this fallback then records the detector over its whole frame. That leaves the transparent child tappable in multi-child detectors. The fallback needs to retain the point-specific traversal result (or otherwise distinguish a bounds miss from a skipped child), rather than using a group-wide any check.

                } else if (view is RNGestureHandlerDetectorView && !allChildrenHidden(view)) {

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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Inline comments:
In
`@packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/GestureHandlerOrchestrator.kt`:
- Around line 916-918: Update allChildrenHidden to evaluate descendant
reachability recursively rather than only checking each direct child with
canReceiveEvents. Ensure a visible ViewGroup with no recursively reachable
descendants is treated as hidden, while groups containing any reachable
descendant keep the detector fallback enabled for recordViewHandlersForPointer
and its hit-slop behavior.

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📒 Files selected for processing (2)

• packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/core/GestureHandlerOrchestrator.kt
• packages/react-native-gesture-handler/android/src/main/java/com/swmansion/gesturehandler/react/RNGestureHandlerRootHelper.kt

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