Bug 812921 - Land in comm-central Instantbird's changes to chat/ - 4 - Bio 1726 - Part and join system messages are asymmetric, r=clokep.

chat/locales/en-US/irc.properties

@@ -75,6 +75,7 @@ command.whois=%S <nick>: Get information on a user.
 #    These are shown as system messages in the conversation.
 #    %1$S is the nick and %2$S is the nick and host of the user who joined.
 message.join=%1$S [%2$S] entered the room.
+message.rejoined=You have rejoined the room.
 #    %1$S is the nick of who kicked you.
 #    %2$S is message.kicked.reason, if a kick message was given.
 message.kicked.you=You have been kicked by %1$S%2$S.
@@ -90,7 +91,7 @@ message.mode=mode (%1$S %2$S) by %3$S.
 message.nick=%1$S is now known as %2$S.
 #    %S is your new nick.
 message.nick.you=You are now known as %S.
-#    The paramter is the message.parted.reason, if a part message is given.
+#    The parameter is the message.parted.reason, if a part message is given.
 message.parted.you=You have left the room (Part%1$S).
 #    %1$S is the user's nick, %2$S is message.parted.reason, if a part message is given.
 message.parted=%1$S has left the room (Part%2$S).

chat/protocols/irc/ircBase.jsm

@@ -195,6 +195,7 @@ var ircBase = {
       // JOIN ( <channel> *( "," <channel> ) [ <key> *( "," <key> ) ] ) / "0"
       // Add the buddy to each channel
       for each (let channelName in aMessage.params[0].split(",")) {
+        let convAlreadyExists = this.hasConversation(channelName);
         let conversation = this.getConversation(channelName);
         if (this.normalize(aMessage.nickname, this.userPrefixes) ==
             this.normalize(this._nickname)) {
@@ -204,6 +205,14 @@ var ircBase = {
           conversation.left = false;
           conversation.notifyObservers(conversation, "update-conv-chatleft");
 
+          // If the user parted from this room earlier, confirm the rejoin.
+          // If conversation._chatRoomFields is present, the rejoin was due to
+          // an automatic reconnection, for which we already notify the user.
+          if (convAlreadyExists && !conversation._chatRoomFields) {
+            conversation.writeMessage(aMessage.nickname, _("message.rejoined"),
+                                      {system: true});
+          }
+
           // Ensure chatRoomFields information is available for reconnection.
           let nName = this.normalize(channelName);
           if (hasOwnProperty(this._chatRoomFieldsList, nName)) {