Alert the user to unread notifications in prior versions of rooms

Fixes https://github.com/vector-im/riot-web/issues/8161

src/components/structures/RoomSubList.js

@@ -29,6 +29,7 @@ import { Group } from 'matrix-js-sdk';
 import PropTypes from 'prop-types';
 import RoomTile from "../views/rooms/RoomTile";
 import LazyRenderList from "../views/elements/LazyRenderList";
+import MatrixClientPeg from "../../MatrixClientPeg";
 
 // turn this on for drop & drag console debugging galore
 const debug = false;
@@ -138,6 +139,28 @@ const RoomSubList = React.createClass({
         this.setState(this.state);
     },
 
+    getUnreadNotificationCount: function(room, type=null) {
+        let notificationCount = room.getUnreadNotificationCount(type);
+
+        // Check notification counts in the old room just in case there's some lost
+        // there. We only go one level down to avoid performance issues, and theory
+        // is that 1st generation rooms will have already been read by the 3rd generation.
+        const createEvent = room.currentState.getStateEvents("m.room.create", "");
+        if (createEvent && createEvent.getContent()['predecessor']) {
+            const oldRoomId = createEvent.getContent()['predecessor']['room_id'];
+            const oldRoom = MatrixClientPeg.get().getRoom(oldRoomId);
+            if (oldRoom) {
+                // We only ever care if there's highlights in the old room. No point in
+                // notifying the user for unread messages because they would have extreme
+                // difficulty changing their notification preferences away from "All Messages"
+                // and "Noisy".
+                notificationCount += oldRoom.getUnreadNotificationCount("highlight");
+            }
+        }
+
+        return notificationCount;
+    },
+
     makeRoomTile: function(room) {
         return <RoomTile
             room={room}
@@ -146,8 +169,8 @@ const RoomSubList = React.createClass({
             key={room.roomId}
             collapsed={this.props.collapsed || false}
             unread={Unread.doesRoomHaveUnreadMessages(room)}
-            highlight={room.getUnreadNotificationCount('highlight') > 0 || this.props.isInvite}
-            notificationCount={room.getUnreadNotificationCount()}
+            highlight={this.props.isInvite || this.getUnreadNotificationCount(room, 'highlight') > 0}
+            notificationCount={this.getUnreadNotificationCount(room)}
             isInvite={this.props.isInvite}
             refreshSubList={this._updateSubListCount}
             incomingCall={null}