compose_box: Hide topic input on a general chat only channel

Related: #1604
Fixes: #1843

Author: chungwwei

lib/widgets/compose_box.dart

@@ -1509,10 +1509,21 @@ class _StreamComposeBoxBody extends _ComposeBoxBody {
   @override
   final StreamComposeBoxController controller;
 
-  @override Widget? buildTopicInput(BuildContext context) => _TopicInput(
-    streamId: narrow.streamId,
-    controller: controller,
-  );
+  @override
+  Widget? buildTopicInput(BuildContext context) {
+    final store = PerAccountStoreWidget.of(context);
+    final stream = store.streams[narrow.streamId];
+    final topicsPolicy = stream?.topicsPolicy;
+
+    if (topicsPolicy == TopicsPolicy.emptyTopicOnly) {
+      return null;
+    }
+
+    return _TopicInput(
+      streamId: narrow.streamId,
+      controller: controller,
+    );
+  }
 
   @override Widget buildContentInput() => _StreamContentInput(
     narrow: narrow,

test/widgets/compose_box_test.dart

@@ -2360,6 +2360,30 @@ void main() {
     // testCancel(narrow: topicNarrow,   start: _EditInteractionStart.restoreFailedEdit);
     // testCancel(narrow: dmNarrow,      start: _EditInteractionStart.restoreFailedEdit);
   });
+
+  group('compose box inputs visibility by topic policy', () {
+    void testComposeBoxWidgetVisibility({required TopicsPolicy topicsPolicy, required bool shouldShowTopicInput}) {
+      final description = shouldShowTopicInput ?
+        'show topic input when topic policy is $topicsPolicy':
+        'hide topic input when topic policy is $topicsPolicy';
+
+      testWidgets(description, (tester) async {
+        final channel = eg.stream(topicsPolicy: topicsPolicy);
+
+        await prepareComposeBox(tester, narrow: ChannelNarrow(channel.streamId), streams: [channel]);
+
+        check(contentInputFinder).findsOne();
+        shouldShowTopicInput ?
+          check(topicInputFinder).findsOne():
+          check(topicInputFinder).findsNothing();
+      });
+    }
+
+    testComposeBoxWidgetVisibility(topicsPolicy: TopicsPolicy.inherit, shouldShowTopicInput: true);
+    testComposeBoxWidgetVisibility(topicsPolicy: TopicsPolicy.allowEmptyTopic, shouldShowTopicInput: true);
+    testComposeBoxWidgetVisibility(topicsPolicy: TopicsPolicy.disableEmptyTopic, shouldShowTopicInput: true);
+    testComposeBoxWidgetVisibility(topicsPolicy: TopicsPolicy.emptyTopicOnly, shouldShowTopicInput: false);
+  });
 }
 
 /// How the edit interaction is started: