Fix tst_QWidgetRepaintManager on XCB

Due to an XCB library change, QXcbBackingStore::toImage() cannot be
safely assumed to return an image identical to QWidget::grab().
The test functions fastMove(), moveAccross() and moveInOutOverlapped()
relied on QXcbBackingStore::toImage() and failed.
They were backlisted on Linux/XCB.

This patch obtains a screen shot instead of an image from the backing
store on XCB platforms. It processes events until the screen shot
matches QWidget::grab(). It makes the test fail only if the comparison
times out.
The patch also removes the BLACKLIST file, containing only the test
functions mentioned above.

Fixes: QTBUG-109036
Change-Id: I26dd5b89dc62b313db066a285f6ad7d4d92baaf2
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 4096667d6601dcbc5e713e6b0fd5b5218453c4cb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/widgets/kernel/qwidgetrepaintmanager/BLACKLIST

@@ -1,11 +0,0 @@
-# Temporary for QTBUG-109036
-[fastMove]
-opensuse-leap
-
-# Temporary for QTBUG-109036
-[moveAccross]
-opensuse-leap
-
-# Temporary for QTBUG-109036
-[moveInOutOverlapped]
-opensuse-leap

tests/auto/widgets/kernel/qwidgetrepaintmanager/tst_qwidgetrepaintmanager.cpp

@@ -262,6 +262,8 @@ private slots:
     void moveInOutOverlapped();
 
 protected:
+    bool m_xcb = false;
+
     /*
         This helper compares the widget as rendered into the backingstore with the widget
         as rendered via QWidget::grab. The latter always produces a fully rendered image,
@@ -279,14 +281,44 @@ protected:
         }
 
         QImage backingstoreContent = platformBackingStore->toImage();
+
+        // QXcbBackingStore::toImage() is not reliable. Take a screenshot instead.
+        // X11 draws the screen in a thread, which is not a QThread.
+        // The first screenshot is therefore unlikely to contain the correct image.
+        // It is taken just to obtain size and format.
+        QWidget *window = nullptr;
+        if (m_xcb) {
+
+            // Widget must be shown to be caught on a screen shot
+            if (!w->isWindow()) {
+                qWarning() << "Hidden widget" << w << "cannot be compared on XCB";
+                return false;
+            }
+
+            window = w->window();
+            Q_ASSERT(window);
+
+            backingstoreContent = window->screen()->grabWindow(window->winId()).toImage();
+        }
+
         if (!w->isWindow()) {
             const qreal dpr = w->devicePixelRatioF();
             const QPointF offset = w->mapTo(w->window(), QPointF(0, 0)) * dpr;
             backingstoreContent = backingstoreContent.copy(offset.x(), offset.y(), w->width() * dpr, w->height() * dpr);
         }
+
         const QImage widgetRender = w->grab().toImage().convertToFormat(backingstoreContent.format());
 
-        const bool result = backingstoreContent == widgetRender;
+        // XCB: Process events, until screenshot equals widgetRender or timeout kicks in.
+        if (m_xcb) {
+            // Unuse result to hit MANUAL_DEBUG code path
+            Q_UNUSED(QTest::qWaitFor([&](){
+                backingstoreContent = window->screen()->grabWindow(window->winId()).toImage();
+                return widgetRender == backingstoreContent;
+            }));
+        }
+
+        const bool result = widgetRender == backingstoreContent;
 
 #ifdef MANUAL_DEBUG
         if (!result) {
@@ -334,6 +366,10 @@ void tst_QWidgetRepaintManager::initTestCase()
 
     m_implementsScroll = widget.backingStore()->handle()->scroll(QRegion(widget.rect()), 1, 1);
     qInfo() << QGuiApplication::platformName() << "QPA backend implements scroll:" << m_implementsScroll;
+
+#if defined(QT_BUILD_INTERNAL)
+    m_xcb = QGuiApplication::platformName().contains("xcb", Qt::CaseInsensitive);
+#endif
 }
 
 void tst_QWidgetRepaintManager::cleanup()