widgets/qpa: Fix painting to a fully transparent top level widget

QWS used to have a line to change the composite mode from SourceOver
to Source for the top level widget. This wasn't used with QPA and I
removed the internal DontSetCompositionMode in qtbase. It turns out that
the QWS way is the most efficient one to initialize the background of
the widget.

The alternative is to have the QPlatformBackingStore::beginPaint
always clear the entire to be painted area and then paint the background
of the widget. The difference of painting each pixel once or twice is
noticable on embedded platforms and in the range of one to two fps.

This does come from Qt5. The change removes the hasFeature test as
CompositionMode_Source is available for all backends.

Reproduce the issue with:
echo "QWidget {background: transparent}" > style.css
./examples/widgets/wiggly/wiggly -stylesheet style.css

Task-number: QTBUG-24526
Change-Id: I3e3f8a263cd3cf9dec8628ca8a3bb28c70572121
Original-Id: Ica4c980bb3bf6eb87ddb5b510ac7493292d01543
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>

1 files changed, 13 insertions, 5 deletions

diff --git a/src/gui/kernel/qwidget.cpp b/src/gui/kernel/qwidget.cpp
index eea7b7c..030e31b 100644
--- a/src/gui/kernel/qwidget.cpp
+++ b/src/gui/kernel/qwidget.cpp
@@ -2420,11 +2420,19 @@ void QWidgetPrivate::paintBackground(QPainter *painter, const QRegion &rgn, int
 
     if ((flags & DrawAsRoot) && !(q->autoFillBackground() && autoFillBrush.isOpaque())) {
         const QBrush bg = q->palette().brush(QPalette::Window);
-#ifdef Q_WS_QWS
-        if (!(flags & DontSetCompositionMode) && painter->paintEngine()->hasFeature(QPaintEngine::PorterDuff))
-            painter->setCompositionMode(QPainter::CompositionMode_Source); //copy alpha straight in
-#endif
+#if defined(Q_WS_QWS) || defined(Q_WS_QPA)
+        if (!(flags & DontSetCompositionMode)) {
+            //copy alpha straight in
+            QPainter::CompositionMode oldMode = painter->compositionMode();
+            painter->setCompositionMode(QPainter::CompositionMode_Source);
+            fillRegion(painter, rgn, bg);
+            painter->setCompositionMode(oldMode);
+        } else {
+            fillRegion(painter, rgn, bg);
+        }
+#else
         fillRegion(painter, rgn, bg);
+#endif
     }
 
     if (q->autoFillBackground())
@@ -5707,7 +5715,7 @@ void QWidgetPrivate::render(QPaintDevice *target, const QPoint &targetOffset,
     else
         flags |= DontSubtractOpaqueChildren;
 
-#ifdef Q_WS_QWS
+#if defined(Q_WS_QWS) || defined(Q_WS_QPA)
     flags |= DontSetCompositionMode;
 #endif