activation: Fix activation notification of Xwayland clients

Most xwayland clients don't know their desktop file name, so use the
StartupWMClass field in their desktop files as means to inferring their
desktop file name.

BUG: 455265

src/xdgactivationv1.cpp

@@ -17,6 +17,7 @@
 #include "wayland_server.h"
 #include "window.h"
 #include "workspace.h"
+#include <KApplicationTrader>
 #include <KDesktopFile>
 
 using namespace KWaylandServer;
@@ -31,6 +32,25 @@ static bool isPrivilegedInWindowManagement(const ClientConnection *client)
     return requestedInterfaces.contains(QLatin1String("org_kde_plasma_window_management"));
 }
 
+static const QString windowDesktopFileName(Window *window)
+{
+    QString ret = window->desktopFileName();
+    if (!ret.isEmpty()) {
+        return ret;
+    }
+
+    // Fallback to StartupWMClass for legacy apps
+    const auto resourceName = window->resourceName();
+    const auto service = KApplicationTrader::query([&resourceName](const KService::Ptr &service) {
+        return service->property("StartupWMClass") == resourceName;
+    });
+
+    if (!service.isEmpty()) {
+        ret = service.constFirst()->desktopEntryName();
+    }
+    return ret;
+}
+
 XdgActivationV1Integration::XdgActivationV1Integration(XdgActivationV1Interface *activation, QObject *parent)
     : QObject(parent)
 {
@@ -41,7 +61,7 @@ XdgActivationV1Integration::XdgActivationV1Integration(XdgActivationV1Interface
         }
 
         // We check that it's not the app that we are trying to activate
-        if (window->desktopFileName() != m_currentActivationToken->applicationId) {
+        if (windowDesktopFileName(window) != m_currentActivationToken->applicationId) {
             // But also that the new one has been requested after the token was requested
             if (window->lastUsageSerial() < m_currentActivationToken->serial) {
                 return;