Expose setDesiredSize to the C++ API

Making it possible for clients to call setDesiredSize directly,

The idea is that under Wayland, the panel calls this intead of setGeometry,
not trying to set an abosulute geometry that might cause the panel sized wrongly,
but just sets an hint which will ensure the panel won't overlap another one

used by https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/5437

CCBUG:489703

src/interfaces/window.cpp

@@ -32,6 +32,7 @@ public:
     Window::KeyboardInteractivity keyboardInteractivity = Window::KeyboardInteractivityOnDemand;
     Window::Layer layer = Window::LayerTop;
     QMargins margins;
+    QSize desiredSize = QSize(0, 0);
     Window::ScreenConfiguration screenConfiguration = Window::ScreenFromQWindow;
     bool closeOnDismissed = true;
 };
@@ -97,6 +98,21 @@ QMargins Window::margins() const
     return d->margins;
 }
 
+void Window::setDesiredSize(const QSize &size)
+{
+    if (size == d->desiredSize) {
+        return;
+    }
+
+    d->desiredSize = size;
+    Q_EMIT desiredSizeChanged();
+}
+
+QSize Window::desiredSize() const
+{
+    return d->desiredSize;
+}
+
 void Window::setKeyboardInteractivity(KeyboardInteractivity interactivity)
 {
     if (d->keyboardInteractivity != interactivity) {

src/interfaces/window.h

@@ -86,6 +86,9 @@ public:
     void setMargins(const QMargins &margins);
     QMargins margins() const;
 
+    void setDesiredSize(const QSize &size);
+    QSize desiredSize() const;
+
     void setKeyboardInteractivity(KeyboardInteractivity interactivity);
     KeyboardInteractivity keyboardInteractivity() const;
 
@@ -127,6 +130,7 @@ Q_SIGNALS:
     void exclusionZoneChanged();
     void exclusiveEdgeChanged();
     void marginsChanged();
+    void desiredSizeChanged();
     void keyboardInteractivityChanged();
     void layerChanged();
 

src/qwaylandlayersurface.cpp

@@ -44,7 +44,11 @@ QWaylandLayerSurface::QWaylandLayerSurface(QWaylandLayerShellIntegration *shell,
     setAnchor(m_interface->anchors());
     connect(m_interface, &Window::anchorsChanged, this, [this]() {
         setAnchor(m_interface->anchors());
+        if (m_interface->desiredSize().isNull()) {
             setDesiredSize(m_window->windowContentGeometry().size());
+        } else {
+            setDesiredSize(m_interface->desiredSize());
+        }
     });
 
     setExclusiveZone(m_interface->exclusionZone());
@@ -61,12 +65,22 @@ QWaylandLayerSurface::QWaylandLayerSurface(QWaylandLayerShellIntegration *shell,
         setMargins(m_interface->margins());
     });
 
+    connect(m_interface, &Window::desiredSizeChanged, this, [this]() {
+        if (!m_interface->desiredSize().isNull()) {
+            setDesiredSize(m_interface->desiredSize());
+        }
+    });
+
     setKeyboardInteractivity(m_interface->keyboardInteractivity());
     connect(m_interface, &Window::keyboardInteractivityChanged, this, [this]() {
         setKeyboardInteractivity(m_interface->keyboardInteractivity());
     });
 
+    if (m_interface->desiredSize().isNull()) {
         setDesiredSize(window->windowContentGeometry().size());
+    } else {
+        setDesiredSize(m_interface->desiredSize());
+    }
 }
 
 QWaylandLayerSurface::~QWaylandLayerSurface()
@@ -174,13 +188,17 @@ void QWaylandLayerSurface::setWindowGeometry(const QRect &geometry)
         return;
     }
 
+    if (m_interface->desiredSize().isNull()) {
         setDesiredSize(geometry.size());
     }
+}
 #else
 void QWaylandLayerSurface::setWindowSize(const QSize &size)
 {
+    if (m_interface->desiredSize().isNull()) {
         setDesiredSize(size);
     }
+}
 #endif
 
 bool QWaylandLayerSurface::requestActivate()