Show a message bar warning on startup if a wayland session is detected

Even on the latest Qt 6 versions, wayland support is limited. Eg. basic
things like re-docking floating docks, moving toolbars, switching
between menus is broken.

Let's show a warning on startup to advise users to switch to an X11
session for the best user experience, so at least they don't place
the blame on QGIS and rather know where to target their bug reports
correctly!

src/app/qgisapp.cpp

@@ -1974,6 +1974,43 @@ QgisApp::QgisApp( QSplashScreen *splash, bool restorePlugins, bool skipBadLayers
   QgsApplication::fontManager()->enableFontDownloadsForSession();
 
   mDevToolsWidget->setActiveTab( lastDevToolsTab );
+
+  if ( QGuiApplication::platformName() == QLatin1String( "wayland" ) )
+  {
+    const bool displayWaylandWarning = settings.value( QStringLiteral( "/UI/displayWaylandWarning" ), true ).toBool();
+    if ( displayWaylandWarning )
+    {
+      const QString shortMessage = tr( "Wayland session detected: User experience will be degraded" );
+      QgsMessageBarItem *messageWidget = QgsMessageBar::createMessage( QString(), shortMessage );
+
+      QPushButton *detailsButton = new QPushButton( tr( "More Info" ) );
+      connect( detailsButton, &QPushButton::clicked, this, [detailsButton]
+      {
+        QgsMessageViewer *dialog = new QgsMessageViewer( detailsButton );
+        dialog->setTitle( tr( "Wayland Session Detected" ) );
+        // NOTE: black coloring MUST be specified here or the message shows white-on-white on wayland sessions 🙃
+        const QString warning = QStringLiteral( "<p style=\"color: black\">%1</p><p style=\"color: black\"><b>%2</b></p>" ).arg(
+          tr( "Running QGIS in a Wayland session will result "
+              "in a degraded experience due to limitations in the "
+              "underlying Qt library and current versions of the Wayland protocol." ),
+          tr( "It is highly recommended that you switch to a traditional X11 session "
+              "for an optimal user experience." ) );
+        dialog->setMessageAsHtml( warning );
+        dialog->showMessage();
+      } );
+      messageWidget->layout()->addWidget( detailsButton );
+
+      QPushButton *ignoreButton = new QPushButton( tr( "Ignore" ) );
+      connect( ignoreButton, &QPushButton::clicked, this, [this, messageWidget]
+      {
+        QgsSettings().setValue( QStringLiteral( "/UI/displayWaylandWarning" ), false );
+        messageBar()->popWidget( messageWidget );
+      } );
+      messageWidget->layout()->addWidget( ignoreButton );
+
+      messageBar()->pushWidget( messageWidget, Qgis::MessageLevel::Critical, 0 );
+    }
+  }
 }
 
 QgisApp::QgisApp()