new logout warning which is easy to see in the background

app/session/session_controller.rb

@@ -0,0 +1,5 @@
+class SessionController < ActionController::Base
+  def logout_warning
+
+  end
+end

app/views/session/_warn_logout.js.erb

@@ -0,0 +1,14 @@
+// window.parent == OpenProject window
+var $ = window.parent.jQuery
+
+$.ajax({
+    url: "/session/logout_warning",
+    cache: false,
+    success: function(html) {
+      $("#logout-warning").remove();
+      $(html).prependTo("body").hide().slideDown();
+      $('html, body').animate({
+        scrollTop: $("#logout-warning").offset().top
+      }, 1000);
+    }
+});

app/views/session/logout_warning.html.erb

@@ -0,0 +1,6 @@
+<div id="logout-warning" class="top-shelf icon icon-warning">
+  <h1><%= t('title.warning') %></h1>
+  <p>
+    <%= t('logout_warning') %>
+  </p>
+</div>

config/locales/de.yml

@@ -0,0 +1,4 @@
+de:
+  logout_warning: >
+    Sie wurden ausgeloggt. Inhalte von Formularen, die sie abschicken möchten,
+    können verloren gehen. Bitte loggen Sie sich wieder ein.

config/locales/en.yml

@@ -0,0 +1,4 @@
+en:
+  logout_warning: >
+    You have been logged out. The contents of any form you submit may be lost.
+    Please log in.

config/routes.rb

@@ -2,4 +2,6 @@ require 'lobby_boy'
 
 Rails.application.routes.draw do
   mount LobbyBoy::Engine, at: '/'
+
+  get '/session/logout_warning', to: 'session#logout_warning'
 end

lib/open_project/openid_connect/lobby_boy_configuration.rb

@@ -36,7 +36,8 @@ module OpenProject
 
         def update_client!
           LobbyBoy.configure_client! host: host,
-                                     end_session_endpoint: end_session_endpoint
+                                     end_session_endpoint: end_session_endpoint,
+                                     on_logout_js_partial: 'session/warn_logout.js'
         end
 
         def update_provider!