make showKeyboard respect finder's skipOffstage value (#79034)

packages/flutter_test/lib/src/widget_tester.dart

@@ -1031,7 +1031,7 @@ class WidgetTester extends WidgetController implements HitTestDispatcher, Ticker
       final EditableTextState editable = state<EditableTextState>(
         find.descendant(
           of: finder,
-          matching: find.byType(EditableText),
+          matching: find.byType(EditableText, skipOffstage: finder.skipOffstage),
           matchRoot: true,
         ),
       );

packages/flutter_test/test/widget_tester_test.dart

@@ -611,7 +611,8 @@ void main() {
     });
   });
 
-  testWidgets('showKeyboard can be called twice', (WidgetTester tester) async {
+  group('showKeyboard', () {
+    testWidgets('can be called twice', (WidgetTester tester) async {
       await tester.pumpWidget(
         MaterialApp(
           home: Material(
@@ -632,6 +633,22 @@ void main() {
       await tester.pump();
     });
 
+    testWidgets(
+      'can focus on offstage text input field if finder says not to skip offstage nodes',
+      (WidgetTester tester) async {
+        await tester.pumpWidget(
+          MaterialApp(
+            home: Material(
+              child: Offstage(
+                child: TextFormField(),
+              ),
+            ),
+          ),
+        );
+        await tester.showKeyboard(find.byType(TextField, skipOffstage: false));
+      });
+  });
+
   testWidgets('verifyTickersWereDisposed control test', (WidgetTester tester) async {
     late FlutterError error;
     final Ticker ticker = tester.createTicker((Duration duration) {});