Resize backingstore properly on devicePixelRatio changes

Task-number: QTBUG-45076
Change-Id: Ib2f2408abf7bd0b6d1c8a05fbbdc665e77ea6392
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

1 files changed, 17 insertions, 6 deletions

diff --git a/src/plugins/platforms/xcb/qxcbbackingstore.cpp b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
index ce2329724a..5c9293c03d 100644
--- a/src/plugins/platforms/xcb/qxcbbackingstore.cpp
+++ b/src/plugins/platforms/xcb/qxcbbackingstore.cpp
@@ -310,7 +310,13 @@ void QXcbBackingStore::beginPaint(const QRegion &region)
 {
     if (!m_image)
         return;
-    const int dpr = int(m_image->image()->devicePixelRatio());
+    int dpr = int(m_image->image()->devicePixelRatio());
+    const int windowDpr = int(window()->devicePixelRatio());
+    if (windowDpr != dpr) {
+        resize(window()->size(), QRegion());
+        dpr = int(m_image->image()->devicePixelRatio());
+    }
+
     QRegion xRegion =  dpr == 1 ? region : QTransform::fromScale(dpr,dpr).map(region);
     m_image->preparePaint(xRegion);
 
@@ -342,7 +348,15 @@ void QXcbBackingStore::flush(QWindow *window, const QRegion &region, const QPoin
     if (!m_image || m_image->size().isEmpty())
         return;
 
-    QSize imageSize = m_image->size();
+    const int dpr = int(window->devicePixelRatio());
+
+#ifndef QT_NO_DEBUG
+    const int imageDpr = int(m_image->image()->devicePixelRatio());
+    if (dpr != imageDpr)
+        qWarning() <<  "QXcbBackingStore::flush() wrong devicePixelRatio for backingstore image" << dpr << imageDpr;
+#endif
+
+    QSize imageSize = m_image->size() / dpr; //because we multiply with the DPR later
 
     QRegion clipped = region;
     clipped &= QRect(0, 0, window->width(), window->height());
@@ -361,8 +375,6 @@ void QXcbBackingStore::flush(QWindow *window, const QRegion &region, const QPoin
         return;
     }
 
-    const int dpr = int(window->devicePixelRatio());
-
     QVector<QRect> rects = clipped.rects();
     for (int i = 0; i < rects.size(); ++i) {
         QRect rect = QRect(rects.at(i).topLeft() * dpr, rects.at(i).size() * dpr);
@@ -399,8 +411,7 @@ void QXcbBackingStore::resize(const QSize &size, const QRegion &)
 {
     const int dpr = int(window()->devicePixelRatio());
     const QSize xSize = size * dpr;
-
-    if (m_image && xSize == m_image->size())
+    if (m_image && xSize == m_image->size() && dpr == m_image->image()->devicePixelRatio())
         return;
     Q_XCB_NOOP(connection());