Revert "drop/restore focus when app becomes invisible/visible" (#25390)

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

diff --git a/packages/flutter/lib/src/widgets/app.dart b/packages/flutter/lib/src/widgets/app.dart
index ec824a4..90735ca 100644
--- a/packages/flutter/lib/src/widgets/app.dart
+++ b/packages/flutter/lib/src/widgets/app.dart
@@ -12,7 +12,6 @@
 import 'banner.dart';
 import 'basic.dart';
 import 'binding.dart';
-import 'focus_manager.dart';
 import 'framework.dart';
 import 'localizations.dart';
 import 'media_query.dart';
@@ -731,33 +730,7 @@
   }
 
   @override
-  void didChangeAppLifecycleState(AppLifecycleState state) {
-    switch (defaultTargetPlatform) {
-      case TargetPlatform.iOS:
-        return;
-      case TargetPlatform.android:
-      case TargetPlatform.fuchsia:
-        // When the application moves to the background, any focus nodes
-        // need to lose focus. When the application is restored to the
-        // foreground state, any focused node should regain focus (including)
-        // restoring the keyboard. This is only important in cases where
-        // applications in the background are partially visible.
-        switch (state) {
-          case AppLifecycleState.paused:
-          case AppLifecycleState.suspending:
-          case AppLifecycleState.inactive: {
-            final FocusManager focusManager = WidgetsBinding.instance.focusManager;
-            focusManager.windowDidLoseFocus();
-            break;
-          }
-          case AppLifecycleState.resumed: {
-            final FocusManager focusManager = WidgetsBinding.instance.focusManager;
-            focusManager.windowDidGainFocus();
-            break;
-          }
-        }
-    }
-  }
+  void didChangeAppLifecycleState(AppLifecycleState state) { }
 
   @override
   void didHaveMemoryPressure() { }

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

diff --git a/packages/flutter/lib/src/widgets/focus_manager.dart b/packages/flutter/lib/src/widgets/focus_manager.dart
index fb32f30..3453c26 100644
--- a/packages/flutter/lib/src/widgets/focus_manager.dart
+++ b/packages/flutter/lib/src/widgets/focus_manager.dart
@@ -417,45 +417,6 @@
   final FocusScopeNode rootScope = FocusScopeNode();
 
   FocusNode _currentFocus;
-  bool _windowHasFocus = true;
-  FocusNode _cachedFocus;
-  FocusScopeNode _cachedScope;
-
-  /// Call when the window loses focus.
-  ///
-  /// The currently focused node and containg scope is cached, then the current
-  /// focus is removed. For example, this ensures that cursors do not blink
-  /// while in the background.
-  ///
-  /// This method is safe to call multiple times.
-  void windowDidLoseFocus() {
-    if (!_windowHasFocus) {
-      return;
-    }
-    _windowHasFocus = false;
-    _cachedFocus = _currentFocus;
-    _cachedScope = _cachedFocus._parent;
-    _currentFocus?.unfocus();
-  }
-
-  /// Call when the window regains focus.
-  ///
-  /// If there is a cached focus node from when the window lost focus, this
-  /// method will restore focus to that node. For example, this ensures that
-  /// a focused text node will re-request the keyboard.
-  ///
-  /// This method is safe to call multiple times.
-  void windowDidGainFocus() {
-    if (_windowHasFocus) {
-      return;
-    }
-    _windowHasFocus = true;
-    if (_cachedFocus != null) {
-      _cachedScope._setFocus(_cachedFocus);
-      _cachedFocus = null;
-      _cachedScope = null;
-    }
-  }
 
   void _willDisposeFocusNode(FocusNode node) {
     assert(node != null);
@@ -473,18 +434,16 @@
 
   FocusNode _findNextFocus() {
     FocusScopeNode scope = rootScope;
-    while (scope._firstChild != null) {
+    while (scope._firstChild != null)
       scope = scope._firstChild;
-    }
     return scope._focus;
   }
 
   void _update() {
     _haveScheduledUpdate = false;
     final FocusNode nextFocus = _findNextFocus();
-    if (_currentFocus == nextFocus) {
+    if (_currentFocus == nextFocus)
       return;
-    }
     final FocusNode previousFocus = _currentFocus;
     _currentFocus = nextFocus;
     previousFocus?._notify();

packages/flutter/test/widgets/focus_test.dart

diff --git a/packages/flutter/test/widgets/focus_test.dart b/packages/flutter/test/widgets/focus_test.dart
index 3fd0981..c02633b 100644
--- a/packages/flutter/test/widgets/focus_test.dart
+++ b/packages/flutter/test/widgets/focus_test.dart
@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/foundation.dart';
-import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/widgets.dart';
 
@@ -246,29 +243,4 @@
 
     parentFocusScope.detach();
   });
-
-  testWidgets('Focus is lost/restored when window focus is lost/restored on Fuchsia', (WidgetTester tester) async {
-    debugDefaultTargetPlatformOverride = TargetPlatform.fuchsia;
-    final FocusNode node = FocusNode();
-    await tester.pumpWidget(MaterialApp(
-      home: Scaffold(
-        body: TextField(
-          focusNode: node,
-          autofocus: true,
-        )
-      ),
-    ));
-    expect(node.hasFocus, true);
-    ByteData message = const StringCodec().encodeMessage('AppLifecycleState.inactive');
-    await BinaryMessages.handlePlatformMessage('flutter/lifecycle', message, (_) {});
-    await tester.pump();
-    // Focus is lost when app goes to background.
-    expect(node.hasFocus, false);
-    message = const StringCodec().encodeMessage('AppLifecycleState.resumed');
-    await BinaryMessages.handlePlatformMessage('flutter/lifecycle', message, (_) {});
-    await tester.pump();
-    // Focus is restored.
-    expect(node.hasFocus, true);
-    debugDefaultTargetPlatformOverride = null;
-  });
 }