Fixes blending problem when paiting non-opaque items with cache enabled.

In most cases when partially updating an item, the old content of the
cache needs to be replaced with the new content
(CompositionMode_Source). But in a specific case when using
DeviceCoordinateCache and when the item is transformed, the new content
needs to be blended (CompositionMode_SourceAtop) with the old one to
avoid incorrect fully transparent background.

Autotest included.

Task-number: QTBUG-7863
Reviewed-by: bnilsen

1 files changed, 6 insertions, 0 deletions

diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index 4ee2301..6934abc 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -4285,6 +4285,12 @@ static void _q_paintIntoCache(QPixmap *pix, QGraphicsItem *item, const QRegion &
     if (!subPix.isNull()) {
         // Blit the subpixmap into the main pixmap.
         pixmapPainter.begin(pix);
+        if (item->cacheMode() == QGraphicsItem::DeviceCoordinateCache
+            && itemToPixmap.type() > QTransform::TxTranslate) {
+            pixmapPainter.setCompositionMode(QPainter::CompositionMode_SourceAtop);
+        } else {
+            pixmapPainter.setCompositionMode(QPainter::CompositionMode_Source);
+        }
         pixmapPainter.setClipRegion(pixmapExposed);
         pixmapPainter.drawPixmap(br.topLeft(), subPix);
         pixmapPainter.end();