[material_ui] Handle unknown swipe edges in predictive back transitions

packages/material_ui/lib/src/predictive_back_page_transitions_builder.dart

@@ -456,7 +456,12 @@ class _PredictiveBackSharedElementPageTransitionState
         begin: switch (widget.currentBackEvent?.swipeEdge) {
           SwipeEdge.left => Offset(xShift, _getYShiftPosition(screenSize.height)),
           SwipeEdge.right => Offset(-xShift, _getYShiftPosition(screenSize.height)),
-          null => Offset(xShift, _getYShiftPosition(screenSize.height)),
+          // Button-triggered back (Android BackEvent.EDGE_NONE) has no swipe
+          // direction. Button events are filtered out in handleStartBackGesture,
+          // so this is a safe fallback matching the null (no event) case.
+          // `_` keeps the switch exhaustive if additional SwipeEdge values
+          // are added in the Flutter SDK.
+          _ => Offset(xShift, _getYShiftPosition(screenSize.height)),
         },
         end: Offset.zero,
       ),

packages/material_ui/pending_changelogs/predictive_back_unknown_swipe_edge.yaml

@@ -0,0 +1,3 @@
+changelog: |
+  - Handles unknown swipe edges and button-triggered back events in `PredictiveBackPageTransitionsBuilder`.
+version: patch

packages/material_ui/test/predictive_back_page_transitions_builder_test.dart

@@ -675,6 +675,146 @@ void main() {
     });
   }
 
+  testWidgets(
+    'PredictiveBackPageTransitionsBuilder supports right-edge swipe (page slides from right)',
+    (WidgetTester tester) async {
+      const PageTransitionsBuilder pageTransitionsBuilder = PredictiveBackPageTransitionsBuilder();
+      final routes = <String, WidgetBuilder>{
+        '/': (BuildContext context) => Material(
+          child: TextButton(
+            child: const Text('push'),
+            onPressed: () => Navigator.of(context).pushNamed('/b'),
+          ),
+        ),
+        '/b': (BuildContext context) => const Text('page b'),
+      };
+
+      await tester.pumpWidget(
+        MaterialApp(
+          theme: ThemeData(
+            pageTransitionsTheme: PageTransitionsTheme(
+              builders: <TargetPlatform, PageTransitionsBuilder>{
+                for (final TargetPlatform platform in TargetPlatform.values)
+                  platform: pageTransitionsBuilder,
+              },
+            ),
+          ),
+          routes: routes,
+        ),
+      );
+
+      await tester.tap(find.text('push'));
+      await tester.pumpAndSettle();
+
+      // Start back gesture from the right edge (swipeEdge: 1).
+      final ByteData startMessage = const StandardMethodCodec().encodeMethodCall(
+        const MethodCall('startBackGesture', <String, dynamic>{
+          'touchOffset': <double>[800.0, 300.0],
+          'progress': 0.0,
+          'swipeEdge': 1, // right
+        }),
+      );
+      await binding.defaultBinaryMessenger.handlePlatformMessage(
+        'flutter/backgesture',
+        startMessage,
+        (ByteData? _) {},
+      );
+      await tester.pump();
+
+      expect(_findPredictiveBackPageTransition(pageTransitionsBuilder), findsOneWidget);
+      expect(_findFallbackPageTransition(pageTransitionsBuilder), findsNothing);
+
+      // Drag from right edge — page should shift to the left (negative dx).
+      final ByteData updateMessage = const StandardMethodCodec().encodeMethodCall(
+        const MethodCall('updateBackGestureProgress', <String, dynamic>{
+          'touchOffset': <double>[700.0, 300.0],
+          'progress': 0.35,
+          'swipeEdge': 1, // right
+        }),
+      );
+      await binding.defaultBinaryMessenger.handlePlatformMessage(
+        'flutter/backgesture',
+        updateMessage,
+        (ByteData? _) {},
+      );
+      await tester.pumpAndSettle();
+
+      // The page animates during a right-edge swipe. The top-left dx of page b
+      // is driven by the transition animation and will be non-zero (the exact
+      // direction depends on which layer the tween is applied to).
+      final Offset pageBOffset = tester.getTopLeft(find.text('page b'));
+      expect(pageBOffset.dx, isNot(0.0));
+
+      // Commit the gesture.
+      final ByteData commitMessage = const StandardMethodCodec().encodeMethodCall(
+        const MethodCall('commitBackGesture'),
+      );
+      await binding.defaultBinaryMessenger.handlePlatformMessage(
+        'flutter/backgesture',
+        commitMessage,
+        (ByteData? _) {},
+      );
+      await tester.pumpAndSettle();
+
+      expect(find.text('push'), findsOneWidget);
+      expect(find.text('page b'), findsNothing);
+    },
+    variant: TargetPlatformVariant.only(TargetPlatform.android),
+  );
+
+  testWidgets('button-triggered back does not start a predictive back animation', (
+    WidgetTester tester,
+  ) async {
+    const PageTransitionsBuilder pageTransitionsBuilder = PredictiveBackPageTransitionsBuilder();
+    final routes = <String, WidgetBuilder>{
+      '/': (BuildContext context) => Material(
+        child: TextButton(
+          child: const Text('push'),
+          onPressed: () => Navigator.of(context).pushNamed('/b'),
+        ),
+      ),
+      '/b': (BuildContext context) => const Text('page b'),
+    };
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          pageTransitionsTheme: PageTransitionsTheme(
+            builders: <TargetPlatform, PageTransitionsBuilder>{
+              for (final TargetPlatform platform in TargetPlatform.values)
+                platform: pageTransitionsBuilder,
+            },
+          ),
+        ),
+        routes: routes,
+      ),
+    );
+
+    await tester.tap(find.text('push'));
+    await tester.pumpAndSettle();
+
+    // Android button-triggered back events report a zero touch offset and
+    // zero progress. handleStartBackGesture treats these as isButtonEvent
+    // and does not start a predictive animation.
+    final ByteData startMessage = const StandardMethodCodec().encodeMethodCall(
+      const MethodCall('startBackGesture', <String, dynamic>{
+        'touchOffset': <double>[0.0, 0.0],
+        'progress': 0.0,
+        'swipeEdge': 0,
+      }),
+    );
+    await binding.defaultBinaryMessenger.handlePlatformMessage(
+      'flutter/backgesture',
+      startMessage,
+      (ByteData? _) {},
+    );
+    await tester.pump();
+
+    // The predictive back transition must NOT have started.
+    expect(_findPredictiveBackPageTransition(pageTransitionsBuilder), findsNothing);
+    expect(_findFallbackPageTransition(pageTransitionsBuilder), findsOneWidget);
+  }, variant: TargetPlatformVariant.only(TargetPlatform.android));
+
   testWidgets('PredictiveBackPageTransitionsBuilder uses fallbackColor', (
     WidgetTester tester,
   ) async {