revive the noPaintOnScreen fix on Windows

The original commit message follows.

Fixes:   Fix the windows PaintOnScreen issue once and for all

Details: To allow both the case where X11 people accidentally set
         PaintOnScreen (which should not have any effect on windows)
         and where people set it and subclass with paintEngine() {
         return 0 } to use GDI / DirectX we do this rather nasty hack.

Original commit in Qt4: 07a2f68bd5869152471e4ffc4a63c683ef141ae8
Autotest: tst_QWidget::paintOnScreenPossible

Change-Id: Ifbb7dc4611959be3ecc362c29a1c3436b0e0fa82
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/widgets/kernel/qwidget_qpa.cpp b/src/widgets/kernel/qwidget_qpa.cpp
index c524491d22..ef3d7a16d9 100644
--- a/src/widgets/kernel/qwidget_qpa.cpp
+++ b/src/widgets/kernel/qwidget_qpa.cpp
@@ -980,6 +980,25 @@ void QWidgetPrivate::setWSGeometry(bool dontShow, const QRect &oldRect)
 QPaintEngine *QWidget::paintEngine() const
 {
     qWarning("QWidget::paintEngine: Should no longer be called");
+
+#ifdef Q_OS_WIN
+    // We set this bit which is checked in setAttribute for
+    // Qt::WA_PaintOnScreen. We do this to allow these two scenarios:
+    //
+    // 1. Users accidentally set Qt::WA_PaintOnScreen on X and port to
+    // Windows which would mean suddenly their widgets stop working.
+    //
+    // 2. Users set paint on screen and subclass paintEngine() to
+    // return 0, in which case we have a "hole" in the backingstore
+    // allowing use of GDI or DirectX directly.
+    //
+    // 1 is WRONG, but to minimize silent failures, we have set this
+    // bit to ignore the setAttribute call. 2. needs to be
+    // supported because its our only means of embedding native
+    // graphics stuff.
+    const_cast<QWidgetPrivate *>(d_func())->noPaintOnScreen = 1;
+#endif
+
     return 0; //##### @@@
 }