Fix clipRect interpretation in composited backingstores

Empty clipRect means "clip away completely", not "no clipping".

Task-number: QTBUG-50719
Change-Id: I6a9dd66130716a921fe9fc245582274e3c9718fe
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>

src/gui/painting/qplatformbackingstore.cpp

@@ -263,12 +263,14 @@ static inline QRect toBottomLeftRect(const QRect &topLeftRect, int windowHeight)
 static void blitTextureForWidget(const QPlatformTextureList *textures, int idx, QWindow *window, const QRect &deviceWindowRect,
                                  QOpenGLTextureBlitter *blitter, const QPoint &offset)
 {
+    const QRect clipRect = textures->clipRect(idx);
+    if (clipRect.isEmpty())
+        return;
+
     QRect rectInWindow = textures->geometry(idx);
     // relative to the TLW, not necessarily our window (if the flush is for a native child widget), have to adjust
     rectInWindow.translate(-offset);
-    QRect clipRect = textures->clipRect(idx);
-    if (clipRect.isEmpty())
-        clipRect = QRect(QPoint(0, 0), rectInWindow.size());
+
     const QRect clippedRectInWindow = rectInWindow & clipRect.translated(rectInWindow.topLeft());
     const QRect srcRect = toBottomLeftRect(clipRect, rectInWindow.height());
 

src/platformsupport/platformcompositor/qopenglcompositor.cpp

@@ -177,11 +177,11 @@ static inline QRect toBottomLeftRect(const QRect &topLeftRect, int windowHeight)
 
 static void clippedBlit(const QPlatformTextureList *textures, int idx, const QRect &targetWindowRect, QOpenGLTextureBlitter *blitter)
 {
-    const QRect rectInWindow = textures->geometry(idx);
-    QRect clipRect = textures->clipRect(idx);
+    const QRect clipRect = textures->clipRect(idx);
     if (clipRect.isEmpty())
-        clipRect = QRect(QPoint(0, 0), rectInWindow.size());
+        return;
 
+    const QRect rectInWindow = textures->geometry(idx);
     const QRect clippedRectInWindow = rectInWindow & clipRect.translated(rectInWindow.topLeft());
     const QRect srcRect = toBottomLeftRect(clipRect, rectInWindow.height());
 

tests/manual/qopenglwidget/openglwidget/main.cpp

@@ -181,6 +181,13 @@ int main(int argc, char *argv[])
     glw3->setObjectName("GL widget in scroll area (possibly native)");
     glw3->setFormat(format);
     glw3->setFixedSize(600, 600);
+    OpenGLWidget *glw3child = new OpenGLWidget(0);
+    const float glw3ClearColor[] = { 0.5f, 0.2f, 0.8f };
+    glw3child->setClearColor(glw3ClearColor);
+    glw3child->setObjectName("Child widget of GL Widget in scroll area");
+    glw3child->setFormat(format);
+    glw3child->setParent(glw3);
+    glw3child->setGeometry(500, 500, 100, 100); // lower right corner of parent
     QScrollArea *sa = new QScrollArea;
     sa->setWidget(glw3);
     sa->setMinimumSize(100, 100);

tests/manual/qopenglwidget/openglwidget/openglwidget.cpp

@@ -78,6 +78,8 @@ public:
 
     int m_interval;
     QVector3D m_rotAxis;
+
+    float clearColor[3];
 };
 
 
@@ -85,6 +87,7 @@ OpenGLWidget::OpenGLWidget(int interval, const QVector3D &rotAxis, QWidget *pare
     : QOpenGLWidget(parent)
 {
     d.reset(new OpenGLWidgetPrivate(this));
+    d->clearColor[0] = d->clearColor[1] = d->clearColor[2] = 0.0f;
     d->m_interval = interval;
     d->m_rotAxis = rotAxis;
     if (interval > 0) {
@@ -151,7 +154,7 @@ void OpenGLWidgetPrivate::render()
     const qreal retinaScale = q->devicePixelRatio();
     glViewport(0, 0, width() * retinaScale, height() * retinaScale);
 
-    glClearColor(0.0, 0.0, 0.0, 1.0);
+    glClearColor(clearColor[0], clearColor[1], clearColor[2], 1.0f);
     glClear(GL_COLOR_BUFFER_BIT);
 
     m_program->bind();
@@ -194,3 +197,10 @@ void OpenGLWidgetPrivate::render()
     if (m_interval <= 0)
         q->update();
 }
+
+void OpenGLWidget::setClearColor(const float *c)
+{
+    d->clearColor[0] = c[0];
+    d->clearColor[1] = c[1];
+    d->clearColor[2] = c[2];
+}

tests/manual/qopenglwidget/openglwidget/openglwidget.h

@@ -49,6 +49,8 @@ public:
     void resizeGL(int w, int h);
     void paintGL();
 
+    void setClearColor(const float *c);
+
 private:
     QScopedPointer<OpenGLWidgetPrivate> d;
 };