From f3e4495e4471204768eaf98d5128e92869e3b435 Mon Sep 17 00:00:00 2001
From: Mouse Zhang <mousezhang7@gmail.com>
Date: Mon, 22 May 2023 16:52:13 +0800
Subject: [PATCH] plugins/qpa: initialize buffer in backingstore

BUG: 437062
---
 src/plugins/qpa/backingstore.cpp | 13 +++++++++++++
 src/plugins/qpa/backingstore.h   |  1 +
 2 files changed, 14 insertions(+)

diff --git a/src/plugins/qpa/backingstore.cpp b/src/plugins/qpa/backingstore.cpp
index 55862c3424..89307b73f2 100644
--- a/src/plugins/qpa/backingstore.cpp
+++ b/src/plugins/qpa/backingstore.cpp
@@ -47,6 +47,18 @@ void BackingStore::resize(const QSize &size, const QRegion &staticContents)
     m_frontBuffer.setDevicePixelRatio(devicePixelRatio);
 }
 
+void BackingStore::beginPaint(const QRegion &region)
+{
+    if (m_backBuffer.hasAlphaChannel()) {
+        QPainter p(paintDevice());
+        p.setCompositionMode(QPainter::CompositionMode_Source);
+        const QColor blank = Qt::transparent;
+        for (const QRect &rect : region) {
+            p.fillRect(rect, blank);
+        }
+    }
+}
+
 static QRect scaledRect(const QRect &rect, qreal devicePixelRatio)
 {
     return QRect(rect.topLeft() * devicePixelRatio, rect.size() * devicePixelRatio);
@@ -55,6 +67,7 @@ static QRect scaledRect(const QRect &rect, qreal devicePixelRatio)
 static void blitImage(const QImage &source, QImage &target, const QRegion &region)
 {
     QPainter painter(&target);
+    painter.setCompositionMode(QPainter::CompositionMode_Source);
     for (const QRect &rect : region) {
         painter.drawImage(rect, source, scaledRect(rect, source.devicePixelRatio()));
     }
diff --git a/src/plugins/qpa/backingstore.h b/src/plugins/qpa/backingstore.h
index 09e4672891..a204e4213f 100644
--- a/src/plugins/qpa/backingstore.h
+++ b/src/plugins/qpa/backingstore.h
@@ -27,6 +27,7 @@ public:
     QPaintDevice *paintDevice() override;
     void flush(QWindow *window, const QRegion &region, const QPoint &offset) override;
     void resize(const QSize &size, const QRegion &staticContents) override;
+    void beginPaint(const QRegion &region) override;
 
 private:
     QImage m_backBuffer;