diff --git a/autotests/integration/internal_window.cpp b/autotests/integration/internal_window.cpp
--- a/autotests/integration/internal_window.cpp
+++ b/autotests/integration/internal_window.cpp
@@ -63,6 +63,7 @@
     void testSkipCloseAnimation();
     void testModifierClickUnrestrictedMove();
     void testModifierScroll();
+    void testPopup();
 };
 
 class HelperWindow : public QRasterWindow
@@ -671,6 +672,21 @@
     kwinApp()->platform()->keyboardKeyReleased(KEY_LEFTALT, timestamp++);
 }
 
+void InternalWindowTest::testPopup()
+{
+    QSignalSpy clientAddedSpy(waylandServer(), &WaylandServer::shellClientAdded);
+    QVERIFY(clientAddedSpy.isValid());
+    HelperWindow win;
+    win.setGeometry(0, 0, 100, 100);
+    win.setFlags(win.flags() | Qt::Popup);
+    win.show();
+    QVERIFY(clientAddedSpy.wait());
+    QCOMPARE(clientAddedSpy.count(), 1);
+    auto internalClient = clientAddedSpy.first().first().value<ShellClient*>();
+    QVERIFY(internalClient);
+    QCOMPARE(internalClient->isPopupWindow(), true);
+}
+
 }
 
 WAYLANDTEST_MAIN(KWin::InternalWindowTest)
diff --git a/shell_client.cpp b/shell_client.cpp
--- a/shell_client.cpp
+++ b/shell_client.cpp
@@ -1982,6 +1982,9 @@
     if (Toplevel::isPopupWindow()) {
         return true;
     }
+    if (isInternal() && m_internalWindow) {
+        return m_internalWindow->flags().testFlag(Qt::Popup);
+    }
     if (m_shellSurface != nullptr) {
         return m_shellSurface->isPopup();
     }