Alert the user to unread notifications in prior versions of rooms

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

src/components/structures/RoomView.js

@@ -1515,6 +1515,25 @@ module.exports = React.createClass({
         }
     },
 
+    _getOldRoom: function() {
+        const createEvent = this.state.room.currentState.getStateEvents("m.room.create", "");
+        if (!createEvent || !createEvent.getContent()['predecessor']) return null;
+
+        return MatrixClientPeg.get().getRoom(createEvent.getContent()['predecessor']['room_id']);
+    },
+
+    _getHiddenHighlightCount: function() {
+        const oldRoom = this._getOldRoom();
+        if (!oldRoom) return 0;
+        return oldRoom.getUnreadNotificationCount('highlight');
+    },
+
+    _onHiddenHighlightsClick: function() {
+        const oldRoom = this._getOldRoom();
+        if (!oldRoom) return;
+        dis.dispatch({action: "view_room", room_id: oldRoom.roomId});
+    },
+
     render: function() {
         const RoomHeader = sdk.getComponent('rooms.RoomHeader');
         const MessageComposer = sdk.getComponent('rooms.MessageComposer');
@@ -1673,6 +1692,8 @@ module.exports = React.createClass({
             !MatrixClientPeg.get().getKeyBackupEnabled()
         );
 
+        const hiddenHighlightCount = this._getHiddenHighlightCount();
+
         let aux = null;
         let hideCancel = false;
         if (this.state.forwardingEvent !== null) {
@@ -1713,6 +1734,15 @@ module.exports = React.createClass({
                                 room={this.state.room}
                 />
             );
+        } else if (hiddenHighlightCount > 0) {
+            aux = (
+                <div className="mx_RoomView_auxPanel_hiddenHighlights" onClick={this._onHiddenHighlightsClick}>
+                    {_t(
+                        "You have %(count)s unread notifications in a prior version of this room.",
+                        {count: hiddenHighlightCount},
+                    )}
+                </div>
+            );
         }
 
         const auxPanel = (