Replace Window::desiredOutput with a Window::screenConfiguration enum

To not overlap with QWindow::screen too much, introduce an enum to make it
more clear what the options and the resulting behaviour are.

src/interfaces/window.cpp

@@ -34,7 +34,7 @@ public:
     Window::KeyboardInteractivity keyboardInteractivity = Window::KeyboardInteractivityExclusive;
     Window::Layer layer = Window::LayerTop;
     QMargins margins;
-    std::optional<QPointer<QScreen>> desiredOutput;
+    Window::ScreenConfiguration screenConfiguration = Window::ScreenFromQWindow;
 };
 
 static QMap<QWindow *, Window *> s_map;
@@ -109,19 +109,14 @@ Window::Layer Window::layer() const
     return d->layer;
 }
 
-QScreen *Window::desiredOutput() const
+Window::ScreenConfiguration Window::screenConfiguration() const
 {
-    // Don't use .value_or here to avoid a temporary QPointer
-    if (d->desiredOutput.has_value()) {
-        return d->desiredOutput.value();
+    return d->screenConfiguration;
 }
 
-    return d->parentWindow->screen();
-}
-
-void Window::setDesiredOutput(QScreen *output)
+void Window::setScreenConfiguration(Window::ScreenConfiguration screenConfiguration)
 {
-    d->desiredOutput = output;
+    d->screenConfiguration = screenConfiguration;
 }
 
 #if QT_VERSION < QT_VERSION_CHECK(6, 6, 0)

src/interfaces/window.h

@@ -58,6 +58,17 @@ public:
     };
     Q_ENUM(KeyboardInteractivity)
 
+    /**
+     * This enum type is used to specify which screen to place the surface on.
+     * ScreenFromQWindow (the default) reads QWindow::screen() while ScreenFromCompositor
+     * passes nil and lets the compositor decide.
+     */
+    enum ScreenConfiguration {
+        ScreenFromQWindow = 0,
+        ScreenFromCompositor = 1,
+    };
+    Q_ENUM(ScreenConfiguration)
+
     void setAnchors(Anchors anchor);
     Anchors anchors() const;
 
@@ -73,13 +84,8 @@ public:
     void setLayer(Layer layer);
     Layer layer() const;
 
-    /**
-     * If set, the compositor will try to put the window on the given screen.
-     * If its not set, then the compositor will decide where to put the window.
-     * Under normal circumstances, this should be the active output.
-     */
-    void setDesiredOutput(QScreen *output);
-    QScreen *desiredOutput() const;
+    void setScreenConfiguration(ScreenConfiguration screenConfiguration);
+    ScreenConfiguration screenConfiguration() const;
 
     /**
      * Sets a string based identifier for this window.

src/qwaylandlayersurface.cpp

@@ -23,9 +23,8 @@ QWaylandLayerSurface::QWaylandLayerSurface(QtWayland::zwlr_layer_shell_v1 *shell
     Q_ASSERT(interface);
 
     wl_output *output = nullptr;
-    QScreen *screen = interface->desiredOutput();
-    if (screen) {
-        auto waylandScreen = dynamic_cast<QtWaylandClient::QWaylandScreen *>(screen->handle());
+    if (interface->screenConfiguration() == Window::ScreenFromQWindow) {
+        auto waylandScreen = dynamic_cast<QtWaylandClient::QWaylandScreen *>(window->window()->screen()->handle());
         // Qt will always assign a screen to a window, but if the compositor has no screens available a dummy QScreen object is created
         // this will not cast to a QWaylandScreen
         if (!waylandScreen) {