Fix scheduling repaints in Effect::prePaintScreen()

If a repaint is scheduled in the prePaintScreen() function, we want
it to be applied in the next frame, not the current one.

Currently, it doesn't work like this because prePaintScreen() runs first
then the Compositor gathers repaints and resets them.

This is important to qtquick effects that use qtquick3d as some items in
qtquick3d schedule repaints for the next frame after synchronizing, i.e.
in OffscreenQuickView::update() which is called in prePaintScreen() by
QuickSceneEffect.

src/compositor.cpp

@@ -157,7 +157,10 @@ void Compositor::composite(RenderLoop *renderLoop)
 
     if (superLayer->needsRepaint()) {
         renderLoop->beginPaint();
-        prePaintPass(superLayer);
+
+        QRegion surfaceDamage = primaryLayer->repaints();
+        primaryLayer->resetRepaints();
+        prePaintPass(superLayer, &surfaceDamage);
 
         SurfaceItem *scanoutCandidate = superLayer->delegate()->scanoutCandidate();
         renderLoop->setFullscreenSurface(scanoutCandidate);
@@ -175,10 +178,6 @@ void Compositor::composite(RenderLoop *renderLoop)
         }
 
         if (!directScanout) {
-            QRegion surfaceDamage = primaryLayer->repaints();
-            primaryLayer->resetRepaints();
-            preparePaintPass(superLayer, &surfaceDamage);
-
             if (auto beginInfo = primaryLayer->beginFrame()) {
                 auto &[renderTarget, repaint] = beginInfo.value();
 
@@ -219,12 +218,23 @@ void Compositor::framePass(RenderLayer *layer)
     }
 }
 
-void Compositor::prePaintPass(RenderLayer *layer)
+void Compositor::prePaintPass(RenderLayer *layer, QRegion *damage)
 {
-    layer->delegate()->prePaint();
+    if (const QRegion repaints = layer->repaints(); !repaints.isEmpty()) {
+        *damage += layer->mapToGlobal(repaints);
+        layer->resetRepaints();
+    }
+
+    const QRegion repaints = layer->delegate()->prePaint();
+    if (!repaints.isEmpty()) {
+        *damage += layer->mapToGlobal(repaints);
+    }
+
     const auto sublayers = layer->sublayers();
     for (RenderLayer *sublayer : sublayers) {
-        prePaintPass(sublayer);
+        if (sublayer->isVisible()) {
+            prePaintPass(sublayer, damage);
+        }
     }
 }
 
@@ -232,20 +242,9 @@ void Compositor::postPaintPass(RenderLayer *layer)
 {
     layer->delegate()->postPaint();
     const auto sublayers = layer->sublayers();
-    for (RenderLayer *sublayer : sublayers) {
-        postPaintPass(sublayer);
-    }
-}
-
-void Compositor::preparePaintPass(RenderLayer *layer, QRegion *repaint)
-{
-    // TODO: Cull opaque region.
-    *repaint += layer->mapToGlobal(layer->repaints() + layer->delegate()->repaints());
-    layer->resetRepaints();
-    const auto sublayers = layer->sublayers();
     for (RenderLayer *sublayer : sublayers) {
         if (sublayer->isVisible()) {
-            preparePaintPass(sublayer, repaint);
+            postPaintPass(sublayer);
         }
     }
 }

src/compositor.h

@@ -146,9 +146,8 @@ protected:
     void addSuperLayer(RenderLayer *layer);
     void removeSuperLayer(RenderLayer *layer);
 
-    void prePaintPass(RenderLayer *layer);
+    void prePaintPass(RenderLayer *layer, QRegion *damage);
     void postPaintPass(RenderLayer *layer);
-    void preparePaintPass(RenderLayer *layer, QRegion *repaint);
     void paintPass(RenderLayer *layer, const RenderTarget &renderTarget, const QRegion &region);
     void framePass(RenderLayer *layer);
 

src/core/renderlayerdelegate.cpp

@@ -19,17 +19,13 @@ void RenderLayerDelegate::setLayer(RenderLayer *layer)
     m_layer = layer;
 }
 
-QRegion RenderLayerDelegate::repaints() const
-{
-    return QRegion();
-}
-
 void RenderLayerDelegate::frame()
 {
 }
 
-void RenderLayerDelegate::prePaint()
+QRegion RenderLayerDelegate::prePaint()
 {
+    return QRegion();
 }
 
 void RenderLayerDelegate::postPaint()

src/core/renderlayerdelegate.h

@@ -29,11 +29,6 @@ public:
     RenderLayer *layer() const;
     void setLayer(RenderLayer *layer);
 
-    /**
-     * Returns the repaints schduled for the next frame.
-     */
-    virtual QRegion repaints() const;
-
     /**
      * This function is called by the compositor after compositing the frame.
      */
@@ -43,7 +38,7 @@ public:
      * This function is called by the compositor before starting painting. Reimplement
      * this function to do frame initialization.
      */
-    virtual void prePaint();
+    virtual QRegion prePaint();
 
     /**
      * This function is called by the compositor after finishing painting. Reimplement

src/scene/cursorscene.cpp

@@ -41,10 +41,11 @@ static void resetRepaintsHelper(Item *item, SceneDelegate *delegate)
     }
 }
 
-void CursorScene::prePaint(SceneDelegate *delegate)
+QRegion CursorScene::prePaint(SceneDelegate *delegate)
 {
     resetRepaintsHelper(m_rootItem.get(), delegate);
     m_paintedOutput = delegate->output();
+    return QRegion();
 }
 
 void CursorScene::postPaint()

src/scene/cursorscene.h

@@ -22,7 +22,7 @@ public:
     explicit CursorScene(std::unique_ptr<ItemRenderer> &&renderer);
     ~CursorScene() override;
 
-    void prePaint(SceneDelegate *delegate) override;
+    QRegion prePaint(SceneDelegate *delegate) override;
     void postPaint() override;
     void paint(const RenderTarget &renderTarget, const QRegion &region) override;
 

src/scene/scene.cpp

@@ -24,19 +24,14 @@ SceneDelegate::~SceneDelegate()
     m_scene->removeDelegate(this);
 }
 
-QRegion SceneDelegate::repaints() const
-{
-    return m_scene->damage().translated(-viewport().topLeft());
-}
-
 SurfaceItem *SceneDelegate::scanoutCandidate() const
 {
     return m_scene->scanoutCandidate();
 }
 
-void SceneDelegate::prePaint()
+QRegion SceneDelegate::prePaint()
 {
-    m_scene->prePaint(this);
+    return m_scene->prePaint(this);
 }
 
 void SceneDelegate::postPaint()

src/scene/scene.h

@@ -29,10 +29,9 @@ public:
     qreal scale() const;
     QRect viewport() const;
 
-    QRegion repaints() const override;
     SurfaceItem *scanoutCandidate() const override;
     void frame() override;
-    void prePaint() override;
+    QRegion prePaint() override;
     void postPaint() override;
     void paint(const RenderTarget &renderTarget, const QRegion &region) override;
 
@@ -83,7 +82,7 @@ public:
     void removeDelegate(SceneDelegate *delegate);
 
     virtual SurfaceItem *scanoutCandidate() const;
-    virtual void prePaint(SceneDelegate *delegate) = 0;
+    virtual QRegion prePaint(SceneDelegate *delegate) = 0;
     virtual void postPaint() = 0;
     virtual void paint(const RenderTarget &renderTarget, const QRegion &region) = 0;
     virtual void frame(SceneDelegate *delegate);

src/scene/workspacescene.cpp

@@ -139,11 +139,6 @@ Item *WorkspaceScene::containerItem() const
     return m_containerItem.get();
 }
 
-QRegion WorkspaceScene::damage() const
-{
-    return m_paintContext.damage;
-}
-
 static SurfaceItem *findTopMostSurface(SurfaceItem *item)
 {
     const QList<Item *> children = item->childItems();
@@ -215,7 +210,7 @@ void WorkspaceScene::frame(SceneDelegate *delegate)
     }
 }
 
-void WorkspaceScene::prePaint(SceneDelegate *delegate)
+QRegion WorkspaceScene::prePaint(SceneDelegate *delegate)
 {
     createStackingOrder();
 
@@ -260,6 +255,8 @@ void WorkspaceScene::prePaint(SceneDelegate *delegate)
     } else {
         preparePaintSimpleScreen();
     }
+
+    return m_paintContext.damage.translated(-delegate->viewport().topLeft());
 }
 
 static void resetRepaintsHelper(Item *item, SceneDelegate *delegate)

src/scene/workspacescene.h

@@ -55,9 +55,8 @@ public:
 
     Item *containerItem() const;
 
-    QRegion damage() const override;
     SurfaceItem *scanoutCandidate() const override;
-    void prePaint(SceneDelegate *delegate) override;
+    QRegion prePaint(SceneDelegate *delegate) override;
     void postPaint() override;
     void paint(const RenderTarget &renderTarget, const QRegion &region) override;
     void frame(SceneDelegate *delegate) override;