Translucent background support

Change-Id: I6e7b5869097f579020c7a4439441a63a0771e955
Task-number: QTBUG-55956
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

2 files changed, 10 insertions, 2 deletions

diff --git a/src/client/qwaylandshmbackingstore.cpp b/src/client/qwaylandshmbackingstore.cpp
index 0afdda4c0..045748a16 100644
--- a/src/client/qwaylandshmbackingstore.cpp
+++ b/src/client/qwaylandshmbackingstore.cpp
@@ -171,12 +171,20 @@ QPaintDevice *QWaylandShmBackingStore::paintDevice()
     return contentSurface();
 }
 
-void QWaylandShmBackingStore::beginPaint(const QRegion &)
+void QWaylandShmBackingStore::beginPaint(const QRegion &region)
 {
     mPainting = true;
     ensureSize();
 
     waylandWindow()->setCanResize(false);
+
+    if (mBackBuffer->image()->hasAlphaChannel()) {
+        QPainter p(paintDevice());
+        p.setCompositionMode(QPainter::CompositionMode_Source);
+        const QColor blank = Qt::transparent;
+        for (const QRect &rect : region)
+            p.fillRect(rect, blank);
+    }
 }
 
 void QWaylandShmBackingStore::endPaint()
diff --git a/src/client/qwaylandshmbackingstore_p.h b/src/client/qwaylandshmbackingstore_p.h
index 8564cc9d2..251368efd 100644
--- a/src/client/qwaylandshmbackingstore_p.h
+++ b/src/client/qwaylandshmbackingstore_p.h
@@ -94,7 +94,7 @@ public:
     void flush(QWindow *window, const QRegion &region, const QPoint &offset) override;
     void resize(const QSize &size, const QRegion &staticContents) override;
     void resize(const QSize &size);
-    void beginPaint(const QRegion &) override;
+    void beginPaint(const QRegion &region) override;
     void endPaint() override;
 
     QWaylandAbstractDecoration *windowDecoration() const;