Mark messages read only when visible, on receipt, focus, scroll

- Only mark messages read when scrolling if in focus and visible
- Remove last seen indicator when scrolling to the bottom with scroll
  down button
- Update last seen indicator when we don't already have one and we're
  scrolled up.

FREEBIE

js/models/conversations.js

@@ -71,7 +71,7 @@
         }.bind(this));
     },
 
-    getUnread: function() {
+    getUnread: function(newestUnreadDate) {
         var conversationId = this.id;
         var unreadMessages = new Whisper.MessageCollection();
         return new Promise(function(resolve) {
@@ -83,7 +83,15 @@
                     upper : [conversationId, Number.MAX_VALUE],
                 }
             }).always(function() {
-                resolve(unreadMessages);
+                if (!newestUnreadDate) {
+                    return resolve(unreadMessages);
+                }
+
+                // TODO: look into an index which would allow us to efficiently get the
+                // set of unread messages before a certain date.
+                resolve(unreadMessages.filter(function(message) {
+                    return message.get('received_at') <= newestUnreadDate;
+                }));
             });
         });
 
@@ -291,15 +299,14 @@
         }
     },
 
-    markRead: function() {
+    markRead: function(newestUnreadDate) {
         if (this.get('unreadCount') > 0) {
-            this.save({ unreadCount: 0 });
             var conversationId = this.id;
             Whisper.Notifications.remove(Whisper.Notifications.where({
                 conversationId: conversationId
             }));
 
-            this.getUnread().then(function(unreadMessages) {
+            this.getUnread(newestUnreadDate).then(function(unreadMessages) {
                 var read = unreadMessages.map(function(m) {
                     if (this.messageCollection.get(m.id)) {
                         m = this.messageCollection.get(m.id);
@@ -313,7 +320,16 @@
                         timestamp : m.get('sent_at')
                     };
                 }.bind(this));
+
                 if (read.length > 0) {
+                    var unreadCount = this.get('unreadCount');
+                    unreadCount = unreadCount - read.length;
+                    if (unreadCount < 0) {
+                        console.log('conversation unreadCount went below zero!');
+                        unreadCount = 0;
+                    }
+                    this.save({ unreadCount: unreadCount });
+
                     console.log('Sending', read.length, 'read receipts');
                     textsecure.messaging.syncReadMessages(read);
                 }