compose [nfc]: Comment on max-topic-length check about cutting off short

We have the same comment where we check the content length:

  if (textNormalized.length > kMaxMessageLengthCodePoints)

and it applies here, too; the API doc on max_topic_length in
/register also says it's in Unicode code points:
  https://zulip.com/api/register-queue

And give our max-topic-length variable a more descriptive name,
again like we do with kMaxMessageLengthCodePoints.

Author: chrisbobbe

lib/api/route/messages.dart

@@ -156,7 +156,7 @@ class GetMessagesResult {
 }
 
 // https://zulip.com/api/send-message#parameter-topic
-const int kMaxTopicLength = 60;
+const int kMaxTopicLengthCodePoints = 60;
 
 // https://zulip.com/api/send-message#parameter-content
 const int kMaxMessageLengthCodePoints = 10000;

lib/widgets/compose_box.dart

@@ -85,7 +85,12 @@ class ComposeTopicController extends ComposeController<TopicValidationError> {
     return [
       if (mandatory && textNormalized == kNoTopicTopic)
         TopicValidationError.mandatoryButEmpty,
-      if (textNormalized.length > kMaxTopicLength)
+
+      // textNormalized.length is the number of UTF-16 code units, while the server
+      // API expresses the max in Unicode code points. So this comparison will
+      // be conservative and may cut the user off shorter than necessary.
+      // TODO(#1238) stop cutting off shorter than necessary
+      if (textNormalized.length > kMaxTopicLengthCodePoints)
         TopicValidationError.tooLong,
     ];
   }