diff --git a/autotests/mock_effectshandler.h b/autotests/mock_effectshandler.h
--- a/autotests/mock_effectshandler.h
+++ b/autotests/mock_effectshandler.h
@@ -137,6 +137,9 @@
     KWin::EffectWindow *findWindow(QWindow *w) const override {
         return nullptr;
     }
+    KWin::EffectWindow *findWindow(const QUuid &id) const override {
+        return nullptr;
+    }
     void *getProxy(QString) override {
         return nullptr;
     }
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
@@ -23,6 +23,7 @@
 #include "../../tabbox/tabboxhandler.h"
 
 #include <QIcon>
+#include <QUuid>
 
 namespace KWin
 {
@@ -65,6 +66,10 @@
         return QIcon();
     }
 
+    QUuid internalId() const override {
+        return QUuid{};
+    }
+
 private:
     QString m_caption;
     WId m_wId;
diff --git a/effects.h b/effects.h
--- a/effects.h
+++ b/effects.h
@@ -135,6 +135,7 @@
     EffectWindow* findWindow(WId id) const override;
     EffectWindow* findWindow(KWayland::Server::SurfaceInterface *surf) const override;
     EffectWindow *findWindow(QWindow *w) const override;
+    EffectWindow *findWindow(const QUuid &id) const override;
     EffectWindowList stackingOrder() const override;
     void setElevatedWindow(KWin::EffectWindow* w, bool set) override;
 
diff --git a/effects.cpp b/effects.cpp
--- a/effects.cpp
+++ b/effects.cpp
@@ -1099,6 +1099,16 @@
     return nullptr;
 }
 
+EffectWindow *EffectsHandlerImpl::findWindow(const QUuid &id) const
+{
+    if (const auto client = workspace()->findAbstractClient([&id] (const AbstractClient *c) { return c->internalId() == id; })) {
+        return client->effectWindow();
+    }
+    if (const auto unmanaged = workspace()->findUnmanaged([&id] (const Unmanaged *c) { return c->internalId() == id; })) {
+        return unmanaged->effectWindow();
+    }
+    return nullptr;
+}
 
 EffectWindowList EffectsHandlerImpl::stackingOrder() const
 {
@@ -2023,7 +2033,7 @@
     if (WindowThumbnailItem *thumb = qobject_cast<WindowThumbnailItem*>(item)) {
         insertThumbnail(thumb);
         connect(thumb, SIGNAL(destroyed(QObject*)), SLOT(thumbnailDestroyed(QObject*)));
-        connect(thumb, SIGNAL(wIdChanged(qulonglong)), SLOT(thumbnailTargetChanged()));
+        connect(thumb, &WindowThumbnailItem::wIdChanged, this, &EffectWindowImpl::thumbnailTargetChanged);
     } else if (DesktopThumbnailItem *desktopThumb = qobject_cast<DesktopThumbnailItem*>(item)) {
         m_desktopThumbnails.append(desktopThumb);
         connect(desktopThumb, SIGNAL(destroyed(QObject*)), SLOT(desktopThumbnailDestroyed(QObject*)));
diff --git a/libkwineffects/kwineffects.h b/libkwineffects/kwineffects.h
--- a/libkwineffects/kwineffects.h
+++ b/libkwineffects/kwineffects.h
@@ -1088,6 +1088,13 @@
      * @since 5.16
      **/
     Q_SCRIPTABLE virtual KWin::EffectWindow *findWindow(QWindow *w) const = 0;
+    /**
+     * Finds the EffectWindow for the Toplevel with KWin internal @p id.
+     * If there is no such window @c null is returned.
+     *
+     * @since 5.16
+     **/
+    Q_SCRIPTABLE virtual KWin::EffectWindow *findWindow(const QUuid &id) const = 0;
     virtual EffectWindowList stackingOrder() const = 0;
     // window will be temporarily painted as if being at the top of the stack
     Q_SCRIPTABLE virtual void setElevatedWindow(KWin::EffectWindow* w, bool set) = 0;
diff --git a/tabbox/clientmodel.cpp b/tabbox/clientmodel.cpp
--- a/tabbox/clientmodel.cpp
+++ b/tabbox/clientmodel.cpp
@@ -25,6 +25,7 @@
 #include "tabboxhandler.h"
 // Qt
 #include <QIcon>
+#include <QUuid>
 // TODO: remove with Qt 5, only for HTML escaping the caption
 #include <QTextDocument>
 #include <QTextStream>
@@ -84,7 +85,7 @@
         return tabBox->desktopName(client.data());
     }
     case WIdRole:
-        return qulonglong(client->window());
+        return client->internalId();
     case MinimizedRole:
         return client->isMinimized();
     case CloseableRole:
diff --git a/tabbox/tabbox.h b/tabbox/tabbox.h
--- a/tabbox/tabbox.h
+++ b/tabbox/tabbox.h
@@ -103,6 +103,7 @@
     virtual bool isCloseable() const;
     virtual void close();
     virtual bool isFirstInTabBox() const;
+    QUuid internalId() const override;
 
     AbstractClient* client() const {
         return m_client;
diff --git a/tabbox/tabbox.cpp b/tabbox/tabbox.cpp
--- a/tabbox/tabbox.cpp
+++ b/tabbox/tabbox.cpp
@@ -441,6 +441,11 @@
     return m_client->isFirstInTabBox();
 }
 
+QUuid TabBoxClientImpl::internalId() const
+{
+    return m_client->internalId();
+}
+
 /*********************************************************
 * TabBox
 *********************************************************/
diff --git a/tabbox/tabboxhandler.h b/tabbox/tabboxhandler.h
--- a/tabbox/tabboxhandler.h
+++ b/tabbox/tabboxhandler.h
@@ -395,6 +395,7 @@
     virtual bool isCloseable() const = 0;
     virtual void close() = 0;
     virtual bool isFirstInTabBox() const = 0;
+    virtual QUuid internalId() const = 0;
 };
 
 /**
diff --git a/thumbnailitem.h b/thumbnailitem.h
--- a/thumbnailitem.h
+++ b/thumbnailitem.h
@@ -22,6 +22,7 @@
 #define KWIN_THUMBNAILITEM_H
 
 #include <QPointer>
+#include <QUuid>
 #include <QWeakPointer>
 #include <QQuickPaintedItem>
 
@@ -76,26 +77,26 @@
 class WindowThumbnailItem : public AbstractThumbnailItem
 {
     Q_OBJECT
-    Q_PROPERTY(qulonglong wId READ wId WRITE setWId NOTIFY wIdChanged SCRIPTABLE true)
+    Q_PROPERTY(QUuid wId READ wId WRITE setWId NOTIFY wIdChanged SCRIPTABLE true)
     Q_PROPERTY(KWin::AbstractClient *client READ client WRITE setClient NOTIFY clientChanged)
 public:
     explicit WindowThumbnailItem(QQuickItem *parent = 0);
     virtual ~WindowThumbnailItem();
 
-    qulonglong wId() const {
+    QUuid wId() const {
         return m_wId;
     }
-    void setWId(qulonglong wId);
+    void setWId(const QUuid &wId);
     AbstractClient *client() const;
     void setClient(AbstractClient *client);
     virtual void paint(QPainter *painter);
 Q_SIGNALS:
-    void wIdChanged(qulonglong wid);
+    void wIdChanged(const QUuid &wid);
     void clientChanged();
 protected Q_SLOTS:
     virtual void repaint(KWin::EffectWindow* w);
 private:
-    qulonglong m_wId;
+    QUuid m_wId;
     AbstractClient *m_client;
 };
 
diff --git a/thumbnailitem.cpp b/thumbnailitem.cpp
--- a/thumbnailitem.cpp
+++ b/thumbnailitem.cpp
@@ -77,7 +77,7 @@
             qCDebug(KWIN_CORE) << "No QQuickWindow assigned yet";
             return;
         }
-        if (auto *w = static_cast<EffectWindowImpl*>(effects->findWindow(qw->winId()))) {
+        if (auto *w = static_cast<EffectWindowImpl*>(effects->findWindow(qw))) {
             m_parent = QWeakPointer<EffectWindowImpl>(w);
         }
     }
@@ -132,18 +132,14 @@
 {
 }
 
-void WindowThumbnailItem::setWId(qulonglong wId)
+void WindowThumbnailItem::setWId(const QUuid &wId)
 {
     if (m_wId == wId) {
         return;
     }
     m_wId = wId;
     if (m_wId != 0) {
-        AbstractClient *c = Workspace::self()->findClient(Predicate::WindowMatch, m_wId);
-        if (!c && waylandServer()) {
-            c = waylandServer()->findClient(m_wId);
-        }
-        setClient(c);
+        setClient(workspace()->findAbstractClient([this] (const AbstractClient *c) { return c->internalId() == m_wId; }));
     } else if (m_client) {
         m_client = NULL;
         emit clientChanged();
@@ -158,9 +154,9 @@
     }
     m_client = client;
     if (m_client) {
-        setWId(m_client->windowId());
+        setWId(m_client->internalId());
     } else {
-        setWId(0);
+        setWId({});
     }
     emit clientChanged();
 }
@@ -170,7 +166,7 @@
     if (effects) {
         return;
     }
-    Client *client = Workspace::self()->findClient(Predicate::WindowMatch, m_wId);
+    auto client = workspace()->findAbstractClient([this] (const AbstractClient *c) { return c->internalId() == m_wId; });
     if (!client) {
         return;
     }
@@ -182,7 +178,7 @@
 
 void WindowThumbnailItem::repaint(KWin::EffectWindow *w)
 {
-    if (static_cast<KWin::EffectWindowImpl*>(w)->window()->windowId() == m_wId) {
+    if (static_cast<KWin::EffectWindowImpl*>(w)->window()->internalId() == m_wId) {
         update();
     }
 }