Fix crash when calling QWidget::grab() on a QOpenGLWidget

By avoiding unneeded nested QPainters.

Crash was:
ASSERT: "s" in file /data/sources/qt/qt5/qtbase/src/gui/painting/qpaintengine_raster.cpp, line 2239

s was nullptr because the inner QPainter had called updateState(0), which is then dereferenced by the outer QPainter.

Task-number: QTBUG-61036
Change-Id: I7aad648f805f1abac4d38dfbefa2292da8b52af4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index a2f3fa4a5f..6112d33974 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -5622,13 +5622,15 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 // punch a hole in the backingstore, so the texture will be visible.
                 if (!q->testAttribute(Qt::WA_AlwaysStackOnTop)) {
                     beginBackingStorePainting();
-                    QPainter p(q);
                     if (backingStore) {
+                        QPainter p(q);
                         p.setCompositionMode(QPainter::CompositionMode_Source);
                         p.fillRect(q->rect(), Qt::transparent);
                     } else {
+                        QImage img = grabFramebuffer();
+                        QPainter p(q);
                         // We are not drawing to a backingstore: fall back to QImage
-                        p.drawImage(q->rect(), grabFramebuffer());
+                        p.drawImage(q->rect(), img);
                         skipPaintEvent = true;
                     }
                     endBackingStorePainting();