Fix exception when selecting in TextField (#41326)

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

@@ -164,15 +164,13 @@ class _ToolbarRenderBox extends RenderShiftedBox {
     child.layout(heightConstraint.enforce(enforcedConstraint), parentUsesSize: true,);
     final _ToolbarParentData childParentData = child.parentData;
 
-    final Offset localTopCenter = globalToLocal(Offset(_arrowTipX, _barTopY));
-
     // The local x-coordinate of the center of the toolbar.
     final double lowerBound = child.size.width/2 + _kToolbarScreenPadding;
     final double upperBound = size.width - child.size.width/2 - _kToolbarScreenPadding;
-    final double adjustedCenterX = localTopCenter.dx.clamp(lowerBound, upperBound);
+    final double adjustedCenterX = _arrowTipX.clamp(lowerBound, upperBound);
 
-    childParentData.offset = Offset(adjustedCenterX - child.size.width / 2, localTopCenter.dy);
-    childParentData.arrowXOffsetFromCenter = localTopCenter.dx - adjustedCenterX;
+    childParentData.offset = Offset(adjustedCenterX - child.size.width / 2, _barTopY);
+    childParentData.arrowXOffsetFromCenter = _arrowTipX - adjustedCenterX;
   }
 
   // The path is described in the toolbar's coordinate system.

packages/flutter/test/cupertino/text_field_test.dart

@@ -3276,6 +3276,55 @@ void main() {
       tester.binding.window.physicalSizeTestValue = null;
       tester.binding.window.devicePixelRatioTestValue = null;
     });
+
+    // This is a regression test for
+    // https://github.com/flutter/flutter/issues/37046.
+    testWidgets('No exceptions when showing selection menu inside of nested Navigators', (WidgetTester tester) async {
+      const String testValue = '123456';
+      final TextEditingController controller = TextEditingController(
+        text: testValue,
+      );
+      await tester.pumpWidget(
+        CupertinoApp(
+          home: CupertinoPageScaffold(
+            child: Center(
+              child: Column(
+                children: <Widget>[
+                  Container(
+                    height: 100,
+                    color: CupertinoColors.activeOrange,
+                  ),
+                  Expanded(
+                    child: Navigator(
+                      onGenerateRoute: (_) =>
+                        CupertinoPageRoute<void>(builder: (_) => Container(
+                          child: CupertinoTextField(
+                            controller: controller,
+                          ),
+                        )),
+                    ),
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      );
+
+      // No text selection toolbar.
+      expect(find.byType(CupertinoTextSelectionToolbar), findsNothing);
+
+      // Double tap on the text in the input.
+      await tester.pumpAndSettle();
+      await tester.tapAt(textOffsetToPosition(tester, testValue.length ~/ 2));
+      await tester.pump(const Duration(milliseconds: 100));
+      await tester.tapAt(textOffsetToPosition(tester, testValue.length ~/ 2));
+      await tester.pumpAndSettle();
+
+      // Now the text selection toolbar is showing and there were no exceptions.
+      expect(find.byType(CupertinoTextSelectionToolbar), findsOneWidget);
+      expect(tester.takeException(), null);
+    });
   });
 
   group('textAlignVertical position', () {

packages/flutter/test/cupertino/text_selection_test.dart

@@ -3,8 +3,8 @@
 // found in the LICENSE file.
 
 import 'package:flutter_test/flutter_test.dart';
-import 'package:flutter/widgets.dart';
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/widgets.dart';
 
 void main() {
   group('canSelectAll', () {