Fixes navigation page back drag area for iPhone X (#24375)

* nav fix and test

packages/flutter/lib/src/cupertino/route.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:math';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
@@ -518,13 +519,19 @@ class _CupertinoBackGestureDetectorState<T> extends State<_CupertinoBackGestureD
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasDirectionality(context));
+    // For devices with notches, the drag area needs to be larger on the side
+    // that has the notch.
+    double dragAreaWidth = Directionality.of(context) == TextDirection.ltr ?
+                           MediaQuery.of(context).padding.left :
+                           MediaQuery.of(context).padding.right;
+    dragAreaWidth = max(dragAreaWidth, _kBackGestureWidth);
     return Stack(
       fit: StackFit.passthrough,
       children: <Widget>[
         widget.child,
         PositionedDirectional(
           start: 0.0,
-          width: _kBackGestureWidth,
+          width: dragAreaWidth,
           top: 0.0,
           bottom: 0.0,
           child: Listener(

packages/flutter/test/cupertino/page_test.dart

@@ -265,6 +265,98 @@ void main() {
     expect(find.text('Page 2'), isOnstage);
   });
 
+  testWidgets('test edge swipes work with media query padding (LTR)', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        builder: (BuildContext context, Widget navigator) {
+          return MediaQuery(
+            data: const MediaQueryData(padding: EdgeInsets.only(left: 40)),
+            child: navigator,
+          );
+        },
+        home: const Placeholder(),
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).push(
+      CupertinoPageRoute<void>(
+        builder: (BuildContext context) => const Center(child: Text('Page 1')),
+      ),
+    );
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 400));
+
+    tester.state<NavigatorState>(find.byType(Navigator)).push(
+      CupertinoPageRoute<void>(
+        builder: (BuildContext context) => const Center(child: Text('Page 2')),
+      ),
+    );
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 400));
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    // Now drag from the left edge.
+    final TestGesture gesture = await tester.startGesture(const Offset(35.0, 200.0));
+    await gesture.moveBy(const Offset(300.0, 0.0));
+    await tester.pump();
+    await tester.pumpAndSettle();
+
+    // Page 1 is now visible.
+    expect(find.text('Page 1'), isOnstage);
+    expect(find.text('Page 2'), isOnstage);
+  });
+
+  testWidgets('test edge swipes work with media query padding (RLT)', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        builder: (BuildContext context, Widget navigator) {
+          return Directionality(
+            textDirection: TextDirection.rtl,
+            child: MediaQuery(
+              data: const MediaQueryData(padding: EdgeInsets.only(right: 40)),
+              child: navigator,
+            ),
+          );
+        },
+        home: const Placeholder(),
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).push(
+      CupertinoPageRoute<void>(
+        builder: (BuildContext context) => const Center(child: Text('Page 1')),
+      ),
+    );
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 400));
+
+    tester.state<NavigatorState>(find.byType(Navigator)).push(
+      CupertinoPageRoute<void>(
+        builder: (BuildContext context) => const Center(child: Text('Page 2')),
+      ),
+    );
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 400));
+
+    expect(find.text('Page 1'), findsNothing);
+    expect(find.text('Page 2'), isOnstage);
+
+    // Now drag from the left edge.
+    final TestGesture gesture = await tester.startGesture(const Offset(765.0, 200.0));
+    await gesture.moveBy(const Offset(-300.0, 0.0));
+    await tester.pump();
+    await tester.pumpAndSettle();
+
+    // Page 1 is now visible.
+    expect(find.text('Page 1'), isOnstage);
+    expect(find.text('Page 2'), isOnstage);
+  });
+
   testWidgets('test only edge swipes work (RTL)', (WidgetTester tester) async {
     await tester.pumpWidget(
       CupertinoApp(