diff --git a/autotests/tabbox/mock_tabboxclient.h b/autotests/tabbox/mock_tabboxclient.h
--- a/autotests/tabbox/mock_tabboxclient.h
+++ b/autotests/tabbox/mock_tabboxclient.h
@@ -30,7 +30,7 @@
 class MockTabBoxClient : public TabBox::TabBoxClient
 {
 public:
-    explicit MockTabBoxClient(QString caption, WId id);
+    explicit MockTabBoxClient(QString caption);
     bool isMinimized() const override {
         return false;
     }
@@ -53,9 +53,6 @@
     int width() const override {
         return 100;
     }
-    WId window() const override {
-        return m_wId;
-    }
     int x() const override {
         return 0;
     }
@@ -72,7 +69,6 @@
 
 private:
     QString m_caption;
-    WId m_wId;
 };
 } // namespace KWin
 #endif
diff --git a/autotests/tabbox/mock_tabboxclient.cpp b/autotests/tabbox/mock_tabboxclient.cpp
--- a/autotests/tabbox/mock_tabboxclient.cpp
+++ b/autotests/tabbox/mock_tabboxclient.cpp
@@ -23,10 +23,9 @@
 namespace KWin
 {
 
-MockTabBoxClient::MockTabBoxClient(QString caption, WId id)
+MockTabBoxClient::MockTabBoxClient(QString caption)
     : TabBoxClient()
     , m_caption(caption)
-    , m_wId(id)
 {
 }
 
diff --git a/autotests/tabbox/mock_tabboxhandler.h b/autotests/tabbox/mock_tabboxhandler.h
--- a/autotests/tabbox/mock_tabboxhandler.h
+++ b/autotests/tabbox/mock_tabboxhandler.h
@@ -100,7 +100,7 @@
     }
 
     // mock methods
-    QWeakPointer<TabBox::TabBoxClient> createMockWindow(const QString &caption, WId id);
+    QWeakPointer<TabBox::TabBoxClient> createMockWindow(const QString &caption);
     void closeWindow(TabBox::TabBoxClient *client);
 private:
     QList< QSharedPointer<TabBox::TabBoxClient> > m_windows;
diff --git a/autotests/tabbox/mock_tabboxhandler.cpp b/autotests/tabbox/mock_tabboxhandler.cpp
--- a/autotests/tabbox/mock_tabboxhandler.cpp
+++ b/autotests/tabbox/mock_tabboxhandler.cpp
@@ -100,9 +100,9 @@
     return false;
 }
 
-QWeakPointer< TabBox::TabBoxClient > MockTabBoxHandler::createMockWindow(const QString &caption, WId id)
+QWeakPointer< TabBox::TabBoxClient > MockTabBoxHandler::createMockWindow(const QString &caption)
 {
-    QSharedPointer< TabBox::TabBoxClient > client(new MockTabBoxClient(caption, id));
+    QSharedPointer< TabBox::TabBoxClient > client(new MockTabBoxClient(caption));
     m_windows.append(client);
     m_activeClient = client;
     return QWeakPointer< TabBox::TabBoxClient >(client);
diff --git a/autotests/tabbox/test_tabbox_clientmodel.cpp b/autotests/tabbox/test_tabbox_clientmodel.cpp
--- a/autotests/tabbox/test_tabbox_clientmodel.cpp
+++ b/autotests/tabbox/test_tabbox_clientmodel.cpp
@@ -38,7 +38,7 @@
     clientModel->createClientList();
     QCOMPARE(clientModel->longestCaption(), QString());
     // add a window to the mock
-    tabboxhandler.createMockWindow(QString("test"), 1);
+    tabboxhandler.createMockWindow(QString("test"));
     clientModel->createClientList();
     QCOMPARE(clientModel->longestCaption(), QString("test"));
     // delete the one client in the list
@@ -59,8 +59,8 @@
     clientModel->createClientList();
     QCOMPARE(clientModel->rowCount(), 0);
     // create two windows, rowCount() should go to two
-    QWeakPointer<TabBox::TabBoxClient> client = tabboxhandler.createMockWindow(QString("test"), 1);
-    tabboxhandler.createMockWindow(QString("test2"), 2);
+    QWeakPointer<TabBox::TabBoxClient> client = tabboxhandler.createMockWindow(QString("test"));
+    tabboxhandler.createMockWindow(QString("test2"));
     clientModel->createClientList();
     QCOMPARE(clientModel->rowCount(), 2);
     // let's ensure there is no active client
@@ -76,8 +76,8 @@
     tabboxhandler.setConfig(TabBox::TabBoxConfig());
     TabBox::ClientModel *clientModel = new TabBox::ClientModel(&tabboxhandler);
     // create two windows, rowCount() should go to two
-    QWeakPointer<TabBox::TabBoxClient> client = tabboxhandler.createMockWindow(QString("test"), 1);
-    client = tabboxhandler.createMockWindow(QString("test2"), 2);
+    QWeakPointer<TabBox::TabBoxClient> client = tabboxhandler.createMockWindow(QString("test"));
+    client = tabboxhandler.createMockWindow(QString("test2"));
     clientModel->createClientList();
     QCOMPARE(clientModel->rowCount(), 2);
 
diff --git a/tabbox/tabbox.h b/tabbox/tabbox.h
--- a/tabbox/tabbox.h
+++ b/tabbox/tabbox.h
@@ -93,7 +93,6 @@
 
     QString caption() const override;
     QIcon icon() const override;
-    WId window() const override;
     bool isMinimized() const override;
     int x() const override;
     int y() const override;
diff --git a/tabbox/tabbox.cpp b/tabbox/tabbox.cpp
--- a/tabbox/tabbox.cpp
+++ b/tabbox/tabbox.cpp
@@ -396,11 +396,6 @@
     return m_client->icon();
 }
 
-WId TabBoxClientImpl::window() const
-{
-    return m_client->windowId();
-}
-
 bool TabBoxClientImpl::isMinimized() const
 {
     return m_client->isMinimized();
diff --git a/tabbox/tabboxhandler.h b/tabbox/tabboxhandler.h
--- a/tabbox/tabboxhandler.h
+++ b/tabbox/tabboxhandler.h
@@ -377,10 +377,6 @@
      * @return The icon of the client
      */
     virtual QIcon icon() const = 0;
-    /**
-     * @return The window Id of the client
-     */
-    virtual WId window() const = 0;
     /**
      * @return Minimized state of the client
      */