Clear backingstore for windows with alpha channel.

Fixes painting artifacts in translucent windows.

Change-Id: I89c198e5636c5387e67ad8839d32dffdc0a149cb
Task-number: QTBUG-43017
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

2 files changed, 13 insertions, 0 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoabackingstore.h b/src/plugins/platforms/cocoa/qcocoabackingstore.h
index d012ede5f9..3737584c4c 100644
--- a/src/plugins/platforms/cocoa/qcocoabackingstore.h
+++ b/src/plugins/platforms/cocoa/qcocoabackingstore.h
@@ -54,6 +54,7 @@ public:
     QImage toImage() const;
     void resize (const QSize &size, const QRegion &);
     bool scroll(const QRegion &area, int dx, int dy);
+    void beginPaint(const QRegion &region);
     qreal getBackingStoreDevicePixelRatio();
 
 private:
diff --git a/src/plugins/platforms/cocoa/qcocoabackingstore.mm b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
index cddb960197..6a4f7ed8ee 100644
--- a/src/plugins/platforms/cocoa/qcocoabackingstore.mm
+++ b/src/plugins/platforms/cocoa/qcocoabackingstore.mm
@@ -96,6 +96,18 @@ bool QCocoaBackingStore::scroll(const QRegion &area, int dx, int dy)
     return true;
 }
 
+void QCocoaBackingStore::beginPaint(const QRegion &region)
+{
+    if (m_qImage.hasAlphaChannel()) {
+        QPainter p(&m_qImage);
+        p.setCompositionMode(QPainter::CompositionMode_Source);
+        const QVector<QRect> rects = region.rects();
+        const QColor blank = Qt::transparent;
+        for (QVector<QRect>::const_iterator it = rects.begin(), end = rects.end(); it != end; ++it)
+            p.fillRect(*it, blank);
+    }
+}
+
 qreal QCocoaBackingStore::getBackingStoreDevicePixelRatio()
 {
     return m_qImage.devicePixelRatio();