subject

src/api/modelTypes.js

@@ -551,7 +551,6 @@ type MessageBase = $ReadOnly<{|
   // Properties that behave differently for stream vs. private messages.
   // TODO: Move all these to `PmMessage` and `StreamMessage`.
 
-  subject: string,
   subject_links: $ReadOnlyArray<string>,
 |}>;
 
@@ -588,6 +587,8 @@ export type PmMessage = $ReadOnly<{|
    * The ordering is less well specified; if it matters, sort first.
    */
   display_recipient: $ReadOnlyArray<PmRecipientUser>,
+
+  subject: '',
 |}>;
 
 export type StreamMessage = $ReadOnly<{|
@@ -604,6 +605,8 @@ export type StreamMessage = $ReadOnly<{|
   display_recipient: string,
 
   stream_id: number,
+
+  subject: string,
 |}>;
 
 export type Message = PmMessage | StreamMessage;

src/message/messagesReducer.js

@@ -139,7 +139,12 @@ export default (state: MessagesState = initialState, action: Action): MessagesSt
           oldMessage && {
             ...(oldMessage: M),
             content: action.rendered_content || oldMessage.content,
-            subject: action.subject || oldMessage.subject,
+            subject:
+              /* eslint-disable operator-linebreak */
+              oldMessage.type === 'private'
+                ? ''
+                : // $FlowFixMe - a non-empty string is fine for stream messages
+                  action.subject || oldMessage.subject,
             subject_links: action.subject_links || oldMessage.subject_links,
             edit_history: [
               action.orig_rendered_content