diff --git a/src/plugins/platforms/offscreen/qoffscreencommon.cpp b/src/plugins/platforms/offscreen/qoffscreencommon.cpp
index 923fffb29c6..3fca2302068 100644
--- a/src/plugins/platforms/offscreen/qoffscreencommon.cpp
+++ b/src/plugins/platforms/offscreen/qoffscreencommon.cpp
@@ -148,7 +148,9 @@ void QOffscreenBackingStore::flush(QWindow *window, const QRegion &region, const
 
 void QOffscreenBackingStore::resize(const QSize &size, const QRegion &)
 {
-    QImage::Format format = QGuiApplication::primaryScreen()->handle()->format();
+    QImage::Format format = window()->format().hasAlpha()
+        ? QImage::Format_ARGB32_Premultiplied
+        : QGuiApplication::primaryScreen()->handle()->format();
     if (m_image.size() != size)
         m_image = QImage(size, format);
     clearHash();
@@ -167,6 +169,17 @@ bool QOffscreenBackingStore::scroll(const QRegion &area, int dx, int dy)
     return true;
 }
 
+void QOffscreenBackingStore::beginPaint(const QRegion &region)
+{
+    if (QImage::toPixelFormat(m_image.format()).alphaUsage() == QPixelFormat::UsesAlpha) {
+        QPainter p(&m_image);
+        p.setCompositionMode(QPainter::CompositionMode_Source);
+        const QColor blank = Qt::transparent;
+        for (const QRect &r : region)
+            p.fillRect(r, blank);
+    }
+}
+
 QPixmap QOffscreenBackingStore::grabWindow(WId window, const QRect &rect) const
 {
     QRect area = m_windowAreaHash.value(window, QRect());
diff --git a/src/plugins/platforms/offscreen/qoffscreencommon.h b/src/plugins/platforms/offscreen/qoffscreencommon.h
index a3d62271688..787af87cfff 100644
--- a/src/plugins/platforms/offscreen/qoffscreencommon.h
+++ b/src/plugins/platforms/offscreen/qoffscreencommon.h
@@ -68,6 +68,7 @@ public:
     void flush(QWindow *window, const QRegion &region, const QPoint &offset) override;
     void resize(const QSize &size, const QRegion &staticContents) override;
     bool scroll(const QRegion &area, int dx, int dy) override;
+    void beginPaint(const QRegion &) override;
 
     QPixmap grabWindow(WId window, const QRect &rect) const;
     QImage toImage() const override { return m_image; }