[scene] Discard pixmaps on buffer size change

The surface size is a logical size, which renders it unsuitable for
deciding whether the window pixmap needs to be discarded.

We need to discard window pixmaps when the buffer size changes. That
has a drawback though, discarding textures is kind of an overkill with
linux-dmabuf. However, fixing that would involve changes in kwayland
server that are far from being trivial.

BUG: 422459

scene.cpp

@@ -406,9 +406,9 @@ void Scene::addToplevel(Toplevel *c)
         // TODO(vlad): Is there a more efficient way to manage window pixmap trees?
         connect(monitor, &SubSurfaceMonitor::subSurfaceAdded, w, &Window::discardPixmap);
         connect(monitor, &SubSurfaceMonitor::subSurfaceRemoved, w, &Window::discardPixmap);
-        connect(monitor, &SubSurfaceMonitor::subSurfaceResized, w, &Window::discardPixmap);
         connect(monitor, &SubSurfaceMonitor::subSurfaceMapped, w, &Window::discardPixmap);
         connect(monitor, &SubSurfaceMonitor::subSurfaceUnmapped, w, &Window::discardPixmap);
+        connect(monitor, &SubSurfaceMonitor::subSurfaceBufferSizeChanged, w, &Window::discardPixmap);
 
         connect(monitor, &SubSurfaceMonitor::subSurfaceAdded, w, &Window::discardQuads);
         connect(monitor, &SubSurfaceMonitor::subSurfaceRemoved, w, &Window::discardQuads);
@@ -418,6 +418,7 @@ void Scene::addToplevel(Toplevel *c)
         connect(monitor, &SubSurfaceMonitor::subSurfaceUnmapped, w, &Window::discardQuads);
         connect(monitor, &SubSurfaceMonitor::subSurfaceSurfaceToBufferMatrixChanged, w, &Window::discardQuads);
 
+        connect(c->surface(), &KWaylandServer::SurfaceInterface::bufferSizeChanged, w, &Window::discardPixmap);
         connect(c->surface(), &KWaylandServer::SurfaceInterface::surfaceToBufferMatrixChanged, w, &Window::discardQuads);
     }
 

subsurfacemonitor.cpp

@@ -48,6 +48,8 @@ void SubSurfaceMonitor::registerSubSurface(SubSurfaceInterface *subSurface)
             this, &SubSurfaceMonitor::subSurfaceUnmapped);
     connect(surface, &SurfaceInterface::surfaceToBufferMatrixChanged,
             this, &SubSurfaceMonitor::subSurfaceSurfaceToBufferMatrixChanged);
+    connect(surface, &SurfaceInterface::bufferSizeChanged,
+            this, &SubSurfaceMonitor::subSurfaceBufferSizeChanged);
 
     registerSurface(surface);
 }
@@ -68,6 +70,8 @@ void SubSurfaceMonitor::unregisterSubSurface(SubSurfaceInterface *subSurface)
                this, &SubSurfaceMonitor::subSurfaceUnmapped);
     disconnect(surface, &SurfaceInterface::surfaceToBufferMatrixChanged,
                this, &SubSurfaceMonitor::subSurfaceSurfaceToBufferMatrixChanged);
+    disconnect(surface, &SurfaceInterface::bufferSizeChanged,
+               this, &SubSurfaceMonitor::subSurfaceBufferSizeChanged);
 
     unregisterSurface(surface);
 }

subsurfacemonitor.h

@@ -75,6 +75,10 @@ Q_SIGNALS:
      * and the buffer coordinate space for a sub-surface has changed.
      */
     void subSurfaceSurfaceToBufferMatrixChanged();
+    /**
+     * This signal is emitted when the buffer size of a subsurface has changed.
+     */
+    void subSurfaceBufferSizeChanged();
 
 private:
     void registerSubSurface(KWaylandServer::SubSurfaceInterface *subSurface);