scene: Freeze SurfaceItem tree when the Window is closed

If the wl_surface is unmapped, the compositor should unmap the window.
Most clients don't do it, and instead destroy the wl_surface or the
surface role object or both. A very tiny fraction of clients actually
close the window by unmapping the wl_surface. Either way, it's worth
handling that case because xdg-shell protocol says that the clients are
allowed to do it.

BUG: 478297

src/scene/surfaceitem.cpp

@@ -227,6 +227,10 @@ ContentType SurfaceItem::contentType() const
     return ContentType::None;
 }
 
+void SurfaceItem::freeze()
+{
+}
+
 SurfaceTexture::~SurfaceTexture()
 {
 }

src/scene/surfaceitem.h

@@ -54,6 +54,8 @@ public:
 
     virtual ContentType contentType() const;
 
+    virtual void freeze();
+
 Q_SIGNALS:
     void damaged();
 

src/scene/surfaceitem_wayland.cpp

@@ -168,6 +168,22 @@ ContentType SurfaceItemWayland::contentType() const
     return m_surface ? m_surface->contentType() : ContentType::None;
 }
 
+void SurfaceItemWayland::freeze()
+{
+    if (!m_surface) {
+        return;
+    }
+
+    m_surface->disconnect(this);
+    if (auto subsurface = m_surface->subSurface()) {
+        subsurface->disconnect(this);
+    }
+
+    for (auto &[subsurface, subsurfaceItem] : m_subsurfaces) {
+        subsurfaceItem->freeze();
+    }
+}
+
 void SurfaceItemWayland::handleColorDescriptionChanged()
 {
     setColorDescription(m_surface->colorDescription());

src/scene/surfaceitem_wayland.h

@@ -31,6 +31,7 @@ public:
     QList<QRectF> shape() const override;
     QRegion opaque() const override;
     ContentType contentType() const override;
+    void freeze() override;
 
     SurfaceInterface *surface() const;
 

src/scene/windowitem.cpp

@@ -55,6 +55,8 @@ WindowItem::WindowItem(Window *window, Scene *scene, Item *parent)
     connect(window, &Window::stackingOrderChanged, this, &WindowItem::updateStackingOrder);
     updateStackingOrder();
 
+    connect(window, &Window::closed, this, &WindowItem::freeze);
+
     m_effectWindow = std::make_unique<EffectWindow>(this);
 }
 
@@ -288,6 +290,13 @@ void WindowItem::markDamaged()
     Q_EMIT m_window->damaged(m_window);
 }
 
+void WindowItem::freeze()
+{
+    if (m_surfaceItem) {
+        m_surfaceItem->freeze();
+    }
+}
+
 WindowItemX11::WindowItemX11(X11Window *window, Scene *scene, Item *parent)
     : WindowItem(window, scene, parent)
 {

src/scene/windowitem.h

@@ -74,6 +74,7 @@ private:
     bool computeVisibility() const;
     void updateVisibility();
     void markDamaged();
+    void freeze();
 
     Window *m_window;
     std::unique_ptr<SurfaceItem> m_surfaceItem;