qml network overview: show server height, if lagging

to see how many blocks it is behind

electrum/gui/qml/components/NetworkOverview.qml

@@ -66,6 +66,15 @@ Pane {
                 Label {
                     text: Network.height
                 }
+                Label {
+                    text: qsTr('Server Height:');
+                    color: Material.accentColor
+                    visible: Network.server_height != Network.height
+                }
+                Label {
+                    text: Network.server_height + " (lagging)"
+                    visible: Network.server_height != Network.height
+                }
                 Heading {
                     Layout.columnSpan: 2
                     text: qsTr('Mempool fees')

electrum/gui/qml/qenetwork.py

@@ -20,7 +20,8 @@ class QENetwork(QObject, QtEventListener):
 
     networkUpdated = pyqtSignal()
     blockchainUpdated = pyqtSignal()
-    heightChanged = pyqtSignal([int], arguments=['height'])
+    heightChanged = pyqtSignal([int], arguments=['height'])  # local blockchain height
+    serverHeightChanged = pyqtSignal([int], arguments=['height'])
     proxySet = pyqtSignal()
     proxyChanged = pyqtSignal()
     statusChanged = pyqtSignal()
@@ -52,6 +53,7 @@ class QENetwork(QObject, QtEventListener):
         self._qeconfig = qeconfig
         self._serverListModel = None
         self._height = network.get_local_height()  # init here, update event can take a while
+        self._server_height = network.get_server_height()  # init here, update event can take a while
         self.register_callbacks()
 
         self._qeconfig.useGossipChanged.connect(self.on_gossip_setting_changed)
@@ -94,6 +96,11 @@ class QENetwork(QObject, QtEventListener):
             self._logger.debug('server_status updated: %s' % server_status)
             self._server_status = server_status
             self.statusChanged.emit()
+        server_height = self.network.get_server_height()
+        if self._server_height != server_height:
+            self._logger.debug(f'server_height updated: {server_height}')
+            self._server_height = server_height
+            self.serverHeightChanged.emit(server_height)
         chains = len(self.network.get_blockchains())
         if chains != self._chaintips:
             self._logger.debug('chain tips # changed: %d', chains)
@@ -173,9 +180,13 @@ class QENetwork(QObject, QtEventListener):
             self.network.run_from_another_thread(self.network.stop_gossip())
 
     @pyqtProperty(int, notify=heightChanged)
-    def height(self):
+    def height(self):  # local blockchain height
         return self._height
 
+    @pyqtProperty(int, notify=serverHeightChanged)
+    def server_height(self):
+        return self._server_height
+
     @pyqtProperty(str, notify=statusChanged)
     def server(self):
         return self._server