Fix up flawed TranslucentBackground usage in docs and examples

Following the qtbase fixes, start doing things correctly in the
QQuickWidget examples too. Remove the checkbox and use a --transparent
command line argument (like in hellogl2).

In addition --transparent --no_render_alpha can be used to verify the
most problematic case: when alpha is present and the backingstore contains
semi-transparent pixels in places where the underlying QOpenGLWidget is
opaque. Here the result must still be an opaque pixel. This was previously
ensured by a glColorMask call, now replaced by glBlendFuncSeparate in
QPlatformBackingStore.

Task-number: QTBUG-47276
Change-Id: Ia040f899405f73e95e957becee5df43683af9c39
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

4 files changed, 19 insertions, 29 deletions

diff --git a/examples/quick/quickwidgets/qquickviewcomparison/main.cpp b/examples/quick/quickwidgets/qquickviewcomparison/main.cpp
index bbe97a947f..7e45e42527 100644
--- a/examples/quick/quickwidgets/qquickviewcomparison/main.cpp
+++ b/examples/quick/quickwidgets/qquickviewcomparison/main.cpp
@@ -48,7 +48,13 @@ int main(int argc, char **argv)
 
     QApplication app(argc, argv);
 
-    MainWindow widgetWindow;
+    bool transparency = QCoreApplication::arguments().contains(QStringLiteral("--transparent"));
+    MainWindow widgetWindow(transparency);
+    if (transparency) {
+        widgetWindow.setAttribute(Qt::WA_TranslucentBackground);
+        widgetWindow.setAttribute(Qt::WA_NoSystemBackground, false);
+    }
+
     widgetWindow.resize(1024, 768);
     widgetWindow.show();
 
diff --git a/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.cpp b/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.cpp
index dce89d6b18..078d8e7e03 100644
--- a/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.cpp
+++ b/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.cpp
@@ -47,10 +47,10 @@
 #include <QLabel>
 #include <QQuickItem>
 
-MainWindow::MainWindow()
+MainWindow::MainWindow(bool transparency)
     : m_currentView(0),
       m_currentRootObject(0),
-      m_transparent(false)
+      m_transparent(transparency)
 {
     QVBoxLayout *layout = new QVBoxLayout;
 
@@ -103,13 +103,6 @@ MainWindow::MainWindow()
     connect(m_checkboxOverlayVisible, &QCheckBox::toggled, m_overlayLabel, &QWidget::setVisible);
     layout->addWidget(m_checkboxOverlayVisible);
 
-    m_checkboxTransparent = new QCheckBox(tr("Transparent background in QQuickWidget"));
-    connect(m_radioWidget, &QCheckBox::toggled, m_checkboxTransparent, &QWidget::setEnabled);
-#ifdef Q_OS_LINUX
-    connect(m_checkboxTransparent, &QCheckBox::toggled, this, &MainWindow::onTransparentChanged);
-    layout->addWidget(m_checkboxTransparent);
-#endif
-
     setLayout(layout);
 
     updateView();
@@ -170,10 +163,8 @@ void MainWindow::updateView()
         switchTo(QWidget::createWindowContainer(quickView));
     } else if (m_state == UseWidget) {
         QQuickWidget *quickWidget = new QQuickWidget;
-        if (m_transparent) {
+        if (m_transparent)
             quickWidget->setClearColor(Qt::transparent);
-            quickWidget->setAttribute(Qt::WA_TranslucentBackground);
-        }
         quickWidget->setFormat(m_format);
         quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
         connect(quickWidget, &QQuickWidget::statusChanged, this, &MainWindow::onStatusChangedWidget);
@@ -186,7 +177,8 @@ void MainWindow::updateView()
     if (m_currentRootObject) {
         m_currentRootObject->setProperty("currentText", text);
         m_currentRootObject->setProperty("multisample", m_checkboxMultiSample->isChecked());
-        m_currentRootObject->setProperty("translucency", m_transparent);
+        if (!QCoreApplication::arguments().contains(QStringLiteral("--no_render_alpha")))
+            m_currentRootObject->setProperty("translucency", m_transparent);
     }
 
     m_overlayLabel->raise();
@@ -242,9 +234,3 @@ void MainWindow::onSceneGraphError(QQuickWindow::SceneGraphError error, const QS
 {
     m_labelStatus->setText(tr("Scenegraph error %1: %2").arg(error).arg(message));
 }
-
-void MainWindow::onTransparentChanged(bool enabled)
-{
-    m_transparent = enabled;
-    updateView();
-}
diff --git a/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.h b/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.h
index c58523f675..5b86c93f38 100644
--- a/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.h
+++ b/examples/quick/quickwidgets/qquickviewcomparison/mainwindow.h
@@ -53,7 +53,7 @@ QT_FORWARD_DECLARE_CLASS(QLayout)
 class MainWindow : public QWidget
 {
 public:
-    MainWindow();
+    MainWindow(bool transparency);
 
 protected:
     void resizeEvent(QResizeEvent*);
@@ -63,7 +63,6 @@ private slots:
     void onStatusChangedView(QQuickView::Status status);
     void onStatusChangedWidget(QQuickWidget::Status status);
     void onSceneGraphError(QQuickWindow::SceneGraphError error, const QString &message);
-    void onTransparentChanged(bool enabled);
 
 private:
     void switchTo(QWidget *view);
@@ -86,8 +85,6 @@ private:
 
     QSurfaceFormat m_format;
 
-    QCheckBox *m_checkboxTransparent;
-
     bool m_transparent;
 };
 
diff --git a/src/quickwidgets/qquickwidget.cpp b/src/quickwidgets/qquickwidget.cpp
index 47bcabc115..9cfec370f9 100644
--- a/src/quickwidgets/qquickwidget.cpp
+++ b/src/quickwidgets/qquickwidget.cpp
@@ -345,8 +345,8 @@ QObject *QQuickWidgetPrivate::focusObject()
     This limitation only applies when there are other widgets underneath the QQuickWidget
     inside the same window. Making the window semi-transparent, with other applications
     and the desktop visible in the background, is done in the traditional way: Set
-    Qt::WA_TranslucentBackground and change the Qt Quick Scenegraph's clear color to
-    Qt::transparent via setClearColor().
+    Qt::WA_TranslucentBackground on the top-level window, request an alpha channel, and
+    change the Qt Quick Scenegraph's clear color to Qt::transparent via setClearColor().
 
     \sa {Exposing Attributes of C++ Types to QML}, {Qt Quick Widgets Example}, QQuickView
 */
@@ -1293,9 +1293,10 @@ QImage QQuickWidget::grabFramebuffer() const
 /*!
   Sets the clear \a color. By default this is an opaque color.
 
-  To get a semi- or fully transparent QQuickWidget, call this function with \a
-  color set to Qt::transparent and set the Qt::WA_TranslucentBackground widget
-  attribute.
+  To get a semi-transparent QQuickWidget, call this function with
+  \a color set to Qt::transparent, set the Qt::WA_TranslucentBackground
+  widget attribute on the top-level window, and request an alpha
+  channel via setFormat().
 
   \sa QQuickWindow::setColor()
  */