Fix QOpenGLWidget on Cocoa when used as viewport

Having a QOpenGLWidget as a graphics view viewport was not functioning
on OS X: it was showing incomplete content due to accessing the texture
attached to the framebuffer object before the rendering is complete.

On the normal path, when rendering is done via paintGL(), the flush was
there. When used as a viewport however, this path is not used. The missing
flush is now added for the other case too. For performance reasons, we will
not flush on every paint engine end(). Instead, the flush is deferred until
composition starts.

QGLWidget also featured a weird on-by-default autoFillBackground concept. To
maintain compatibility with apps that used QGLWidget as the viewport for
QGraphicsView, we will now do the same for QOpenGLWidget, but only when it
is used as a viewport. For regular QOpenGLWidgets autoFillBackground defaults
to false, like for any other widget. The docs are extended with a small section
about differences between QGLWidget and QOpenGLWidget.

Task-number: QTBUG-41046
Change-Id: I42c2033fdd2ef5815783fd640fe11373761061e0
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

3 files changed, 32 insertions, 1 deletions

diff --git a/src/gui/opengl/qopenglpaintdevice.cpp b/src/gui/opengl/qopenglpaintdevice.cpp
index 59bca6efdf..c2f3295bc3 100644
--- a/src/gui/opengl/qopenglpaintdevice.cpp
+++ b/src/gui/opengl/qopenglpaintdevice.cpp
@@ -351,14 +351,39 @@ bool QOpenGLPaintDevice::paintFlipped() const
 }
 
 /*!
+  This virtual method is called when starting to paint.
+
+  The default implementation does nothing.
+
+  \sa endPaint()
+ */
+void QOpenGLPaintDevice::beginPaint()
+{
+}
+
+/*!
     This virtual method is provided as a callback to allow re-binding a target
     frame buffer object or context when different QOpenGLPaintDevice instances
     are issuing draw calls alternately.
 
-    QPainter::beginNativePainting will also trigger this method.
+    \l{QPainter::beginNativePainting()}{beginNativePainting()} will also trigger
+    this method.
+
+    The default implementation does nothing.
 */
 void QOpenGLPaintDevice::ensureActiveTarget()
 {
 }
 
+/*!
+  This virtual method is called when the painting has finished.
+
+  The default implementation does nothing.
+
+  \sa beginPaint()
+*/
+void QOpenGLPaintDevice::endPaint()
+{
+}
+
 QT_END_NAMESPACE
diff --git a/src/gui/opengl/qopenglpaintdevice.h b/src/gui/opengl/qopenglpaintdevice.h
index e1be9b525d..d88992d6db 100644
--- a/src/gui/opengl/qopenglpaintdevice.h
+++ b/src/gui/opengl/qopenglpaintdevice.h
@@ -73,7 +73,9 @@ public:
     void setPaintFlipped(bool flipped);
     bool paintFlipped() const;
 
+    virtual void beginPaint();
     virtual void ensureActiveTarget();
+    virtual void endPaint();
 
 protected:
     int metric(QPaintDevice::PaintDeviceMetric metric) const;
diff --git a/src/gui/opengl/qopenglpaintengine.cpp b/src/gui/opengl/qopenglpaintengine.cpp
index 21bc4a95e8..40c836b2bb 100644
--- a/src/gui/opengl/qopenglpaintengine.cpp
+++ b/src/gui/opengl/qopenglpaintengine.cpp
@@ -1994,6 +1994,8 @@ bool QOpenGL2PaintEngineEx::begin(QPaintDevice *pdev)
     d->ctx = QOpenGLContext::currentContext();
     d->ctx->d_func()->active_engine = this;
 
+    d->device->beginPaint();
+
     d->funcs.initializeOpenGLFunctions();
 
     for (int i = 0; i < QT_GL_VERTEX_ARRAY_TRACKED_COUNT; ++i)
@@ -2044,6 +2046,8 @@ bool QOpenGL2PaintEngineEx::end()
 {
     Q_D(QOpenGL2PaintEngineEx);
 
+    d->device->endPaint();
+
     QOpenGLContext *ctx = d->ctx;
     d->funcs.glUseProgram(0);
     d->transferMode(BrushDrawingMode);