action-sheet: Pass the correct BuildContext for showing SnackBar

Before this, the `SnackBar` wouldn't show as the correct `BuildContext` was
not passed to `copyWithPopup` method.

Fixes: #732

Author: sm-sayedi

lib/widgets/action_sheet.dart

@@ -361,7 +361,7 @@ class CopyMessageTextButton extends MessageActionSheetMenuItemButton {
 
     if (!messageListContext.mounted) return;
 
-    copyWithPopup(context: context,
+    copyWithPopup(context: messageListContext,
       successContent: Text(zulipLocalizations.successMessageTextCopied),
       data: ClipboardData(text: rawContent));
   }

test/widgets/action_sheet_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:http/http.dart' as http;
 import 'package:zulip/api/model/model.dart';
 import 'package:zulip/api/route/messages.dart';
+import 'package:zulip/model/binding.dart';
 import 'package:zulip/model/compose.dart';
 import 'package:zulip/model/localizations.dart';
 import 'package:zulip/model/narrow.dart';
@@ -464,6 +465,9 @@ void main() {
     }
 
     testWidgets('success', (WidgetTester tester) async {
+      // for #732 regression check below
+      testBinding.deviceInfoResult = IosDeviceInfo(systemVersion: '16.0');
+
       final message = eg.streamMessage();
       await setupToMessageActionSheet(tester, message: message, narrow: TopicNarrow.ofMessage(message));
       final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
@@ -472,6 +476,14 @@ void main() {
       await tapCopyMessageTextButton(tester);
       await tester.pump(Duration.zero);
       check(await Clipboard.getData('text/plain')).isNotNull().text.equals('Hello world');
+
+      // regression check for #732
+      final snackbar = tester.widget<SnackBar>(find.byType(SnackBar));
+      check(snackbar.behavior).equals(SnackBarBehavior.floating);
+      final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
+      tester.widget(find.descendant(matchRoot: true,
+        of: find.byWidget(snackbar.content),
+        matching: find.text(zulipLocalizations.successMessageTextCopied)));
     });
 
     testWidgets('request has an error', (WidgetTester tester) async {