Allow specifying explicit desired screen

If the window position is not specified, which is a reasonable thing to
do when using the layer shell protocol, setWidth() and setHeight() can
unintentionally change the screen() to the wrong one.

This change adds a setDesiredScreen() function so it's harder to shoot
yourself in the foot while using the layer shell protocol.

src/interfaces/window.cpp

@@ -34,7 +34,8 @@ public:
     Window::Layer layer = Window::LayerTop;
     QMargins margins;
     QSize desiredSize = QSize(0, 0);
-    Window::ScreenConfiguration screenConfiguration = Window::ScreenFromQWindow;
+    QPointer<QScreen> desiredScreen;
+    bool desiredActiveScreen = false;
     bool closeOnDismissed = true;
     bool activateOnShow = true;
 };
@@ -152,14 +153,46 @@ Window::Layer Window::layer() const
     return d->layer;
 }
 
-Window::ScreenConfiguration Window::screenConfiguration() const
+void Window::setDesiredActiveScreen(bool set)
 {
-    return d->screenConfiguration;
+    if (d->desiredActiveScreen == set) {
+        return;
+    }
+
+    d->desiredActiveScreen = set;
+
+    if (d->desiredActiveScreen && d->desiredScreen) {
+        d->desiredScreen = nullptr;
+        Q_EMIT desiredScreenChanged();
+    }
+
+    Q_EMIT desiredActiveScreenChanged();
 }
 
-void Window::setScreenConfiguration(Window::ScreenConfiguration screenConfiguration)
+bool Window::desiredActiveScreen() const
 {
-    d->screenConfiguration = screenConfiguration;
+    return d->desiredActiveScreen;
+}
+
+void Window::setDesiredScreen(QScreen *screen)
+{
+    if (d->desiredScreen == screen) {
+        return;
+    }
+
+    d->desiredScreen = screen;
+
+    if (d->desiredScreen && d->desiredActiveScreen) {
+        d->desiredActiveScreen = false;
+        Q_EMIT desiredActiveScreenChanged();
+    }
+
+    Q_EMIT desiredScreenChanged();
+}
+
+QScreen *Window::desiredScreen() const
+{
+    return d->desiredScreen;
 }
 
 bool Window::closeOnDismissed() const

src/interfaces/window.h

@@ -27,8 +27,9 @@ class LAYERSHELLQT_EXPORT Window : public QObject
     Q_PROPERTY(qint32 exclusionZone READ exclusionZone WRITE setExclusiveZone NOTIFY exclusionZoneChanged)
     Q_PROPERTY(Layer layer READ layer WRITE setLayer NOTIFY layerChanged)
     Q_PROPERTY(KeyboardInteractivity keyboardInteractivity READ keyboardInteractivity WRITE setKeyboardInteractivity NOTIFY keyboardInteractivityChanged)
-    Q_PROPERTY(ScreenConfiguration screenConfiguration READ screenConfiguration WRITE setScreenConfiguration)
     Q_PROPERTY(bool activateOnShow READ activateOnShow WRITE setActivateOnShow)
+    Q_PROPERTY(bool desiredActiveScreen READ desiredActiveScreen WRITE setDesiredActiveScreen NOTIFY desiredActiveScreenChanged)
+    Q_PROPERTY(QScreen *desiredScreen READ desiredScreen WRITE setDesiredScreen NOTIFY desiredScreenChanged)
 
 public:
     ~Window() override;
@@ -64,17 +65,6 @@ 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;
 
@@ -96,8 +86,33 @@ public:
     void setLayer(Layer layer);
     Layer layer() const;
 
-    void setScreenConfiguration(ScreenConfiguration screenConfiguration);
-    ScreenConfiguration screenConfiguration() const;
+    /**
+     * Indicates whether the layer shell surface should be placed on the active screen based on @a set.
+     *
+     * The active screen depends on the compositor policies.
+     *
+     * If no explicit desired screen has been specified with the setDesiredScreen() function or the layer
+     * surface doesn't need to be placed on the active screen, i.e. setDesiredActiveScreen() has not
+     * been called, the QWindow::screen() will be used to decide what screen the layer shell surface
+     * should be placed on.
+     *
+     * The desiredScreen() will be reset if @a set is @c true.
+     */
+    void setDesiredActiveScreen(bool set);
+    bool desiredActiveScreen() const;
+
+    /**
+     * Indicates that the layer shell surface should be placed on the specified @a screen.
+     *
+     * If no explicit desired screen has been specified with the setDesiredScreen() function or the layer
+     * surface doesn't need to be placed on the active screen, i.e. setDesiredActiveScreen() has not
+     * been called, the QWindow::screen() will be used to decide what screen the layer shell surface
+     * should be placed on.
+     *
+     * The desiredActiveScreen() will be reset to @c false after calling this function.
+     */
+    void setDesiredScreen(QScreen *screen);
+    QScreen *desiredScreen() const;
 
     /**
      * Sets a string based identifier for this window.
@@ -148,6 +163,8 @@ Q_SIGNALS:
     void desiredSizeChanged();
     void keyboardInteractivityChanged();
     void layerChanged();
+    void desiredActiveScreenChanged();
+    void desiredScreenChanged();
 
 private:
     void initializeShell();

src/qwaylandlayersurface.cpp

@@ -26,7 +26,12 @@ QWaylandLayerSurface::QWaylandLayerSurface(QWaylandLayerShellIntegration *shell,
     , m_window(window)
 {
     wl_output *output = nullptr;
-    if (m_interface->screenConfiguration() == Window::ScreenFromQWindow) {
+    if (!m_interface->desiredActiveScreen()) {
+        QScreen *desiredScreen = m_interface->desiredScreen();
+        if (!desiredScreen) {
+            window->window()->screen();
+        }
+
         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