diff --git a/decorations/decoratedclient.cpp b/decorations/decoratedclient.cpp
--- a/decorations/decoratedclient.cpp
+++ b/decorations/decoratedclient.cpp
@@ -137,7 +137,7 @@
 DELEGATE(bool, isKeepAbove, keepAbove)
 DELEGATE(bool, isKeepBelow, keepBelow)
 DELEGATE(bool, isShaded, isShade)
-DELEGATE(WId, windowId, window)
+DELEGATE(WId, windowId, windowId)
 DELEGATE(WId, decorationId, frameId)
 
 #undef DELEGATE
diff --git a/effects.cpp b/effects.cpp
--- a/effects.cpp
+++ b/effects.cpp
@@ -166,6 +166,9 @@
 
 static QByteArray readWindowProperty(xcb_window_t win, xcb_atom_t atom, xcb_atom_t type, int format)
 {
+    if (win == XCB_WINDOW_NONE) {
+        return QByteArray();
+    }
     uint32_t len = 32768;
     for (;;) {
         Xcb::Property prop(false, win, atom, XCB_ATOM_ANY, 0, len);
@@ -183,6 +186,9 @@
 
 static void deleteWindowProperty(Window win, long int atom)
 {
+    if (win == XCB_WINDOW_NONE) {
+        return;
+    }
     xcb_delete_property(connection(), win, atom);
 }
 
diff --git a/shadow.cpp b/shadow.cpp
--- a/shadow.cpp
+++ b/shadow.cpp
@@ -127,12 +127,14 @@
 QVector< uint32_t > Shadow::readX11ShadowProperty(xcb_window_t id)
 {
     QVector<uint32_t> ret;
-    Xcb::Property property(false, id, atoms->kde_net_wm_shadow, XCB_ATOM_CARDINAL, 0, 12);
-    uint32_t *shadow = property.value<uint32_t*>();
-    if (shadow) {
-        ret.reserve(12);
-        for (int i=0; i<12; ++i) {
-            ret << shadow[i];
+    if (id != XCB_WINDOW) {
+        Xcb::Property property(false, id, atoms->kde_net_wm_shadow, XCB_ATOM_CARDINAL, 0, 12);
+        uint32_t *shadow = property.value<uint32_t*>();
+        if (shadow) {
+            ret.reserve(12);
+            for (int i=0; i<12; ++i) {
+                ret << shadow[i];
+            }
         }
     }
     return ret;
diff --git a/shell_client.h b/shell_client.h
--- a/shell_client.h
+++ b/shell_client.h
@@ -90,16 +90,15 @@
     bool userCanSetFullScreen() const override;
     bool userCanSetNoBorder() const override;
     bool wantsInput() const override;
-    xcb_window_t window() const override;
     using AbstractClient::resizeWithChecks;
     void resizeWithChecks(int w, int h, ForceGeometry_t force = NormalGeometrySet) override;
     using AbstractClient::setGeometry;
     void setGeometry(int x, int y, int w, int h, ForceGeometry_t force = NormalGeometrySet) override;
     bool hasStrut() const override;
 
     void setInternalFramebufferObject(const QSharedPointer<QOpenGLFramebufferObject> &fbo) override;
 
-    quint32 windowId() const {
+    quint32 windowId() const override {
         return m_windowId;
     }
     bool isInternal() const;
diff --git a/shell_client.cpp b/shell_client.cpp
--- a/shell_client.cpp
+++ b/shell_client.cpp
@@ -766,11 +766,6 @@
     return surface()->client() == waylandServer()->inputMethodConnection();
 }
 
-xcb_window_t ShellClient::window() const
-{
-    return windowId();
-}
-
 void ShellClient::requestGeometry(const QRect &rect)
 {
     m_positionAfterResize.setPoint(rect.topLeft());
diff --git a/tabbox/tabbox.cpp b/tabbox/tabbox.cpp
--- a/tabbox/tabbox.cpp
+++ b/tabbox/tabbox.cpp
@@ -375,7 +375,7 @@
 
 WId TabBoxClientImpl::window() const
 {
-    return m_client->window();
+    return m_client->windowId();
 }
 
 bool TabBoxClientImpl::isMinimized() const
diff --git a/thumbnailitem.cpp b/thumbnailitem.cpp
--- a/thumbnailitem.cpp
+++ b/thumbnailitem.cpp
@@ -159,7 +159,7 @@
     }
     m_client = client;
     if (m_client) {
-        setWId(m_client->window());
+        setWId(m_client->windowId());
     } else {
         setWId(0);
     }
@@ -183,7 +183,7 @@
 
 void WindowThumbnailItem::repaint(KWin::EffectWindow *w)
 {
-    if (static_cast<KWin::EffectWindowImpl*>(w)->window()->window() == m_wId) {
+    if (static_cast<KWin::EffectWindowImpl*>(w)->window()->windowId() == m_wId) {
         update();
     }
 }
diff --git a/toplevel.h b/toplevel.h
--- a/toplevel.h
+++ b/toplevel.h
@@ -82,7 +82,7 @@
     Q_PROPERTY(int screen READ screen NOTIFY screenChanged)
     Q_PROPERTY(QSize size READ size)
     Q_PROPERTY(int width READ width)
-    Q_PROPERTY(qulonglong windowId READ window CONSTANT)
+    Q_PROPERTY(qulonglong windowId READ windowId CONSTANT)
     Q_PROPERTY(int x READ x)
     Q_PROPERTY(int y READ y)
     Q_PROPERTY(int desktop READ desktop)
@@ -213,7 +213,11 @@
 public:
     explicit Toplevel();
     virtual xcb_window_t frameId() const;
-    virtual xcb_window_t window() const;
+    xcb_window_t window() const;
+    /**
+     * @return a unique identifier for the Toplevel. On X11 same as @link {window}
+     **/
+    virtual quint32 windowId() const;
     QRect geometry() const;
     QSize size() const;
     QPoint pos() const;
diff --git a/toplevel.cpp b/toplevel.cpp
--- a/toplevel.cpp
+++ b/toplevel.cpp
@@ -519,5 +519,10 @@
     return m;
 }
 
+quint32 Toplevel::windowId() const
+{
+    return window();
+}
+
 } // namespace
 
diff --git a/useractions.cpp b/useractions.cpp
--- a/useractions.cpp
+++ b/useractions.cpp
@@ -222,7 +222,7 @@
         args << QStringLiteral("--dontagain") << QLatin1String("kwin_dialogsrc:") + type;
     }
     if (!c.isNull())
-        args << QStringLiteral("--embed") << QString::number(c.data()->window());
+        args << QStringLiteral("--embed") << QString::number(c.data()->windowId());
     QtConcurrent::run([args]() {
         KProcess::startDetached(QStringLiteral("kdialog"), args);
     });
@@ -1704,7 +1704,7 @@
     bool succeeded = false;
 
     if (Xcb::Extensions::self()->isRandrAvailable()) {
-        ScreenResources res(active_client ? active_client->window() : rootWindow());
+        ScreenResources res((active_client && active_client->window() != XCB_WINDOW_NONE) ? active_client->window() : rootWindow());
 
         if (!res.isNull()) {
             for (int j = 0; j < res->num_crtcs; ++j) {