kcms/tabbox: Add mock-up object for KWin.DesktopBackground

The window switcher previews don't use the actual quick components
the KWin plugin provides, but mock-up objects of those.

Add a mock-up component for `KWin.DesktopBackground`, which is just
a thumbnail of the desktop default image but stretched to cover the
whole parent size.

This fixes the previews for flip&cover window switchers.

BUG: 479552
FIXED-IN: 5.93.0

src/kcms/tabbox/layoutpreview.cpp

@@ -33,6 +33,7 @@ LayoutPreview::LayoutPreview(const QString &path, bool showDesktopThumbnail, QOb
     QQmlComponent *component = new QQmlComponent(engine, this);
     qmlRegisterType<WindowThumbnailItem>("org.kde.kwin", 3, 0, "WindowThumbnail");
     qmlRegisterType<SwitcherItem>("org.kde.kwin", 3, 0, "TabBoxSwitcher");
+    qmlRegisterType<DesktopBackground>("org.kde.kwin", 3, 0, "DesktopBackground");
     qmlRegisterAnonymousType<QAbstractItemModel>("org.kde.kwin", 3);
     component->loadUrl(QUrl::fromLocalFile(path));
     if (component->isError()) {
@@ -246,6 +247,28 @@ void SwitcherItem::decrementIndex()
     setCurrentIndex(index);
 }
 
+DesktopBackground::DesktopBackground(QQuickItem *parent)
+    : WindowThumbnailItem(parent)
+{
+    setWId(WindowThumbnailItem::Desktop);
+
+    connect(this, &QQuickItem::windowChanged, this, &DesktopBackground::stretchToScreen);
+    stretchToScreen();
+};
+
+void DesktopBackground::stretchToScreen()
+{
+    const QQuickWindow *w = window();
+    if (!w) {
+        return;
+    }
+    const QScreen *screen = w->screen();
+    if (!screen) {
+        return;
+    }
+    setImplicitSize(screen->size().width(), screen->size().height());
+};
+
 } // namespace KWin
 } // namespace TabBox
 

src/kcms/tabbox/layoutpreview.h

@@ -149,5 +149,25 @@ inline QObject *SwitcherItem::item() const
     return m_item;
 }
 
+class DesktopBackground : public WindowThumbnailItem
+{
+    Q_OBJECT
+    Q_PROPERTY(QVariant activity MEMBER m_activity)
+    Q_PROPERTY(QVariant desktop MEMBER m_desktop)
+    Q_PROPERTY(QString outputName MEMBER m_outputName)
+
+public:
+    DesktopBackground(QQuickItem *parent = nullptr);
+
+private Q_SLOTS:
+    void stretchToScreen();
+
+private:
+    // Just for mock-up purposes.
+    QVariant m_activity;
+    QVariant m_desktop;
+    QString m_outputName;
+};
+
 } // namespace TabBox
 } // namespace KWin