DEV: Use online/offline window events to track network connectivity (#22243)

spec/system/network_disconnected_spec.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+RSpec.describe "Network Disconnected", type: :system do
+  def with_network_disconnected
+    page.driver.browser.network_conditions = { offline: true }
+    yield
+    page.driver.browser.network_conditions = { offline: false }
+  end
+
+  it "NetworkConnectivity service adds class to DOM and displays offline indicator" do
+    SiteSetting.enable_offline_indicator = true
+
+    visit("/c")
+
+    expect(page).to have_no_css("html.network-disconnected")
+    expect(page).to have_no_css(".offline-indicator")
+
+    with_network_disconnected do
+      # Message bus connectivity services adds the disconnected class to the DOM
+      expect(page).to have_css("html.network-disconnected")
+
+      # Offline indicator is rendered
+      expect(page).to have_css(".offline-indicator")
+    end
+  end
+end