diff --git a/autotests/integration/shell_client_test.cpp b/autotests/integration/shell_client_test.cpp
--- a/autotests/integration/shell_client_test.cpp
+++ b/autotests/integration/shell_client_test.cpp
@@ -20,6 +20,7 @@
 #include "kwin_wayland_test.h"
 #include "cursor.h"
 #include "effects.h"
+#include "deleted.h"
 #include "platform.h"
 #include "shell_client.h"
 #include "screens.h"
@@ -95,6 +96,7 @@
 
 void TestShellClient::initTestCase()
 {
+    qRegisterMetaType<KWin::Deleted*>();
     qRegisterMetaType<KWin::ShellClient*>();
     qRegisterMetaType<KWin::AbstractClient*>();
     qRegisterMetaType<KWayland::Client::Output*>();
@@ -174,6 +176,12 @@
     QVERIFY(client->property("moveable").toBool());
     QVERIFY(client->property("moveableAcrossScreens").toBool());
     QVERIFY(client->property("resizeable").toBool());
+    QCOMPARE(client->internalId().isNull(), false);
+    const auto uuid = client->internalId();
+    QUuid deletedUuid;
+    QCOMPARE(deletedUuid.isNull(), true);
+
+    connect(client, &ShellClient::windowClosed, this, [&deletedUuid] (Toplevel *, Deleted *d) { deletedUuid = d->internalId(); });
 
     // now unmap
     QSignalSpy hiddenSpy(client, &ShellClient::windowHidden);
@@ -212,6 +220,7 @@
     QCOMPARE(hiddenSpy.count(), 2);
     QCOMPARE(client->readyForPainting(), true);
     QCOMPARE(client->isHiddenInternal(), true);
+    QCOMPARE(client->internalId(), uuid);
     QVERIFY(windowClosedSpy.isEmpty());
     QCOMPARE(effectsWindowHiddenSpy.count(), 2);
     QCOMPARE(effectsWindowHiddenSpy.last().first().value<EffectWindow*>(), client->effectWindow());
@@ -221,6 +230,8 @@
     QVERIFY(windowClosedSpy.wait());
     QCOMPARE(windowClosedSpy.count(), 1);
     QCOMPARE(effectsWindowHiddenSpy.count(), 2);
+    QCOMPARE(deletedUuid.isNull(), false);
+    QCOMPARE(deletedUuid, uuid);
 }
 
 void TestShellClient::testDesktopPresenceChanged()
diff --git a/autotests/integration/x11_client_test.cpp b/autotests/integration/x11_client_test.cpp
--- a/autotests/integration/x11_client_test.cpp
+++ b/autotests/integration/x11_client_test.cpp
@@ -24,6 +24,7 @@
 #include "effects.h"
 #include "effectloader.h"
 #include "cursor.h"
+#include "deleted.h"
 #include "platform.h"
 #include "screens.h"
 #include "shell_client.h"
@@ -60,6 +61,7 @@
 
 void X11ClientTest::initTestCase()
 {
+    qRegisterMetaType<KWin::Deleted*>();
     qRegisterMetaType<KWin::ShellClient*>();
     qRegisterMetaType<KWin::AbstractClient*>();
     QSignalSpy workspaceCreatedSpy(kwinApp(), &Application::workspaceCreated);
@@ -352,6 +354,13 @@
     QCOMPARE(client->windowId(), w);
     QVERIFY(client->isActive());
     QCOMPARE(client->window(), w);
+    QCOMPARE(client->internalId().isNull(), false);
+    const auto uuid = client->internalId();
+    QUuid deletedUuid;
+    QCOMPARE(deletedUuid.isNull(), true);
+
+    connect(client, &Client::windowClosed, this, [&deletedUuid] (Toplevel *, Deleted *d) { deletedUuid = d->internalId(); });
+
 
     NETRootInfo rootInfo(c.data(), NET::WMAllProperties);
     QCOMPARE(rootInfo.activeWindow(), client->window());
@@ -379,6 +388,12 @@
     // and destroy the window again
     xcb_unmap_window(c.data(), w);
     xcb_flush(c.data());
+    QSignalSpy windowClosedSpy(client, &Client::windowClosed);
+    QVERIFY(windowClosedSpy.isValid());
+    QVERIFY(windowClosedSpy.wait());
+
+    QCOMPARE(deletedUuid.isNull(), false);
+    QCOMPARE(deletedUuid, uuid);
 }
 
 void X11ClientTest::testCaptionChanges()
diff --git a/toplevel.h b/toplevel.h
--- a/toplevel.h
+++ b/toplevel.h
@@ -31,6 +31,7 @@
 // Qt
 #include <QObject>
 #include <QMatrix4x4>
+#include <QUuid>
 // xcb
 #include <xcb/damage.h>
 #include <xcb/xfixes.h>
@@ -458,6 +459,14 @@
      **/
     virtual bool isPopupWindow() const;
 
+    /**
+     * A UUID to uniquely identify this Toplevel independent of windowing system.
+     **/
+    QUuid internalId() const
+    {
+        return m_internalId;
+    }
+
 Q_SIGNALS:
     void opacityChanged(KWin::Toplevel* toplevel, qreal oldOpacity);
     void damaged(KWin::Toplevel* toplevel, const QRect& damage);
@@ -577,6 +586,7 @@
 
 private:
     // when adding new data members, check also copyToDeleted()
+    QUuid m_internalId;
     Xcb::Window m_client;
     xcb_damage_damage_t damage_handle;
     QRegion damage_region; // damage is really damaged window (XDamage) and texture needs
diff --git a/toplevel.cpp b/toplevel.cpp
--- a/toplevel.cpp
+++ b/toplevel.cpp
@@ -44,6 +44,7 @@
     , info(NULL)
     , ready_for_painting(true)
     , m_isDamaged(false)
+    , m_internalId(QUuid::createUuid())
     , m_client()
     , damage_handle(None)
     , is_shape(false)
@@ -107,6 +108,7 @@
 // used only by Deleted::copy()
 void Toplevel::copyToDeleted(Toplevel* c)
 {
+    m_internalId = c->internalId();
     geom = c->geom;
     m_visual = c->m_visual;
     bit_depth = c->bit_depth;