Fix the issue of TextSelection could misbehave inside a Nested Overlay (#49257)

packages/flutter/lib/src/widgets/overlay.dart

@@ -242,8 +242,18 @@ class Overlay extends StatefulWidget {
   /// ```dart
   /// OverlayState overlay = Overlay.of(context);
   /// ```
-  static OverlayState of(BuildContext context, { Widget debugRequiredFor }) {
-    final OverlayState result = context.findAncestorStateOfType<OverlayState>();
+  ///
+  /// If `rootOverlay` is set to true, the state from the furthest instance of
+  /// this class is given instead. Useful for installing overlay entries
+  /// above all subsequent instances of [Overlay].
+  static OverlayState of(
+    BuildContext context, {
+    bool rootOverlay = false,
+    Widget debugRequiredFor,
+  }) {
+    final OverlayState result = rootOverlay
+        ? context.findRootAncestorStateOfType<OverlayState>()
+        : context.findAncestorStateOfType<OverlayState>();
     assert(() {
       if (debugRequiredFor != null && result == null) {
         final List<DiagnosticsNode> information = <DiagnosticsNode>[

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

@@ -283,7 +283,7 @@ class TextSelectionOverlay {
        assert(handlesVisible != null),
        _handlesVisible = handlesVisible,
        _value = value {
-    final OverlayState overlay = Overlay.of(context);
+    final OverlayState overlay = Overlay.of(context, rootOverlay: true);
     assert(overlay != null,
       'No Overlay widget exists above $context.\n'
       'Usually the Navigator created by WidgetsApp provides the overlay. Perhaps your '
@@ -410,7 +410,7 @@ class TextSelectionOverlay {
 
 
 
-    Overlay.of(context, debugRequiredFor: debugRequiredFor).insertAll(_handles);
+    Overlay.of(context, rootOverlay: true, debugRequiredFor: debugRequiredFor).insertAll(_handles);
   }
 
   /// Destroys the handles by removing them from overlay.
@@ -426,7 +426,7 @@ class TextSelectionOverlay {
   void showToolbar() {
     assert(_toolbar == null);
     _toolbar = OverlayEntry(builder: _buildToolbar);
-    Overlay.of(context, debugRequiredFor: debugRequiredFor).insert(_toolbar);
+    Overlay.of(context, rootOverlay: true, debugRequiredFor: debugRequiredFor).insert(_toolbar);
     _toolbarController.forward(from: 0.0);
   }
 

packages/flutter/test/material/text_field_test.dart

@@ -7432,4 +7432,45 @@ void main() {
 
     expect(textFieldSize1, equals(textFieldSize2));
   });
+
+  testWidgets(
+    'The selection menu displays in an Overlay without error',
+    (WidgetTester tester) async {
+      // This is a regression test for
+      // https://github.com/flutter/flutter/issues/43787
+      final TextEditingController controller = TextEditingController(
+        text: 'This is a test that shows some odd behavior with Text Selection!',
+      );
+
+      await tester.pumpWidget(MaterialApp(
+        home: Scaffold(
+          body: Container(
+            color: Colors.grey,
+            child: Center(
+              child: Container(
+                color: Colors.red,
+                width: 300,
+                height: 600,
+                child: Overlay(
+                  initialEntries: <OverlayEntry>[
+                    OverlayEntry(
+                      builder: (BuildContext context) => Center(
+                        child: TextField(
+                          controller: controller,
+                        ),
+                      ),
+                    )
+                  ],
+                ),
+              ),
+            ),
+          ),
+        ),
+      ));
+
+      await _showSelectionMenuAt(tester, controller, controller.text.indexOf('test'));
+      await tester.pumpAndSettle();
+      expect(tester.takeException(), isNull);
+    },
+  );
 }