Revert optimizations and fixes for moving/scrolling overlapped widgets

This reverts the QtWidgets changes made in commits
22634e00794e72d68e7578e1962f9f2023870749 and
5b09346cf4322704a866f253b911d467c40df3ba while keeping the auto tests
introduced in the former commit.

Both commits introduced rendering errors when moving widgets out of
or into areas in which they are obscured. Before we apply any further
optimizations to this code we need thorough auto test coverage.

Task-number: QTBUG-98151
Task-number: QTBUG-26269
Pick-to: 6.2
Change-Id: I9cb82b73776daed59ea0e9f51ff7ddef1c7265b6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

2 files changed, 28 insertions, 80 deletions

diff --git a/src/widgets/kernel/qwidget_p.h b/src/widgets/kernel/qwidget_p.h
index db458be7f1..41f343dfec 100644
--- a/src/widgets/kernel/qwidget_p.h
+++ b/src/widgets/kernel/qwidget_p.h
@@ -390,6 +390,7 @@ public:
     void invalidateBackingStore(const T &);
 
     QRegion overlappedRegion(const QRect &rect, bool breakAfterFirst = false) const;
+    bool isOverlapped(const QRect &rect) const { return !overlappedRegion(rect, true).isEmpty(); }
     void syncBackingStore();
     void syncBackingStore(const QRegion &region);
 
diff --git a/src/widgets/kernel/qwidgetrepaintmanager.cpp b/src/widgets/kernel/qwidgetrepaintmanager.cpp
index d9375c7281..734d3424fd 100644
--- a/src/widgets/kernel/qwidgetrepaintmanager.cpp
+++ b/src/widgets/kernel/qwidgetrepaintmanager.cpp
@@ -414,25 +414,6 @@ static bool hasPlatformWindow(QWidget *widget)
     return widget && widget->windowHandle() && widget->windowHandle()->handle();
 }
 
-static QList<QRect> getSortedRectsToScroll(const QRegion &region, int dx, int dy)
-{
-    QList<QRect> rects;
-    std::copy(region.begin(), region.end(), std::back_inserter(rects));
-    if (rects.count() > 1) {
-        std::sort(rects.begin(), rects.end(), [=](const QRect &r1, const QRect &r2) {
-            if (r1.y() == r2.y()) {
-                if (dx > 0)
-                    return r1.x() > r2.x();
-                return r1.x() < r2.x();
-            }
-            if (dy > 0)
-                return r1.y() > r2.y();
-            return r1.y() < r2.y();
-        });
-    }
-    return rects;
-}
-
 //parent's coordinates; move whole rect; update parent and widget
 //assume the screen blt has already been done, so we don't need to refresh that part
 void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
@@ -449,17 +430,21 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
     QWidget *pw = q->parentWidget();
     QPoint toplevelOffset = pw->mapTo(tlw, QPoint());
     QWidgetPrivate *pd = pw->d_func();
-    const QRect clipR(pd->clipRect());
+    QRect clipR(pd->clipRect());
     const QRect newRect(rect.translated(dx, dy));
+    QRect destRect = rect.intersected(clipR);
+    if (destRect.isValid())
+        destRect = destRect.translated(dx, dy).intersected(clipR);
+    const QRect sourceRect(destRect.translated(-dx, -dy));
     const QRect parentRect(rect & clipR);
     const bool nativeWithTextureChild = textureChildSeen && hasPlatformWindow(q);
 
-    const bool accelerateMove = accelEnv && isOpaque && !nativeWithTextureChild
+    bool accelerateMove = accelEnv && isOpaque && !nativeWithTextureChild
 #if QT_CONFIG(graphicsview)
                           // No accelerate move for proxy widgets.
                           && !tlw->d_func()->extra->proxyWidget
 #endif
-            ;
+                          && !isOverlapped(sourceRect) && !isOverlapped(destRect);
 
     if (!accelerateMove) {
         QRegion parentR(effectiveRectFor(parentRect));
@@ -472,50 +457,22 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
         pd->invalidateBackingStore(parentR);
         invalidateBackingStore((newRect & clipR).translated(-data.crect.topLeft()));
     } else {
-        QRect destRect = rect.intersected(clipR);
-        if (destRect.isValid())
-            destRect = destRect.translated(dx, dy).intersected(clipR);
-        const QRect sourceRect(destRect.translated(-dx, -dy));
 
         QWidgetRepaintManager *repaintManager = x->repaintManager.get();
-        QRegion childExpose = QRegion(newRect) & clipR;
-        QRegion overlappedExpose;
+        QRegion childExpose(newRect & clipR);
 
-        if (sourceRect.isValid()) {
-            overlappedExpose = (overlappedRegion(sourceRect) | overlappedRegion(destRect)) & clipR;
-
-            const qreal factor = QHighDpiScaling::factor(q->windowHandle());
-            if (overlappedExpose.isEmpty() || qFloor(factor) == factor) {
-                const QList<QRect> rectsToScroll =
-                        getSortedRectsToScroll(QRegion(sourceRect) - overlappedExpose, dx, dy);
-                for (QRect r : rectsToScroll) {
-                    if (repaintManager->bltRect(r, dx, dy, pw)) {
-                        childExpose -= r.translated(dx, dy);
-                    }
-                }
-                isMoved = true;
-            }
-
-            childExpose -= overlappedExpose;
-        }
+        if (sourceRect.isValid() && repaintManager->bltRect(sourceRect, dx, dy, pw))
+            childExpose -= destRect;
 
         if (!pw->updatesEnabled())
             return;
 
         const bool childUpdatesEnabled = q->updatesEnabled();
-        if (childUpdatesEnabled) {
-            // As per paintAndFlush, reset isMoved if we have overlapping
-            // or child regions that need to be painted.
-            if (!overlappedExpose.isEmpty()) {
-                overlappedExpose.translate(-data.crect.topLeft());
-                invalidateBackingStore(overlappedExpose);
-                isMoved = false;
-            }
-            if (!childExpose.isEmpty()) {
-                childExpose.translate(-data.crect.topLeft());
-                repaintManager->markDirty(childExpose, q);
-                isMoved = false;
-            }
+
+        if (childUpdatesEnabled && !childExpose.isEmpty()) {
+            childExpose.translate(-data.crect.topLeft());
+            repaintManager->markDirty(childExpose, q);
+            isMoved = true;
         }
 
         QRegion parentExpose(parentRect);
@@ -523,10 +480,12 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
         if (extra && extra->hasMask)
             parentExpose += QRegion(newRect) - extra->mask.translated(data.crect.topLeft());
 
-        if (!parentExpose.isEmpty())
+        if (!parentExpose.isEmpty()) {
             repaintManager->markDirty(parentExpose, pw);
+            pd->isMoved = true;
+        }
 
-        if (childUpdatesEnabled && sourceRect.isValid()) {
+        if (childUpdatesEnabled) {
             QRegion needsFlush(sourceRect);
             needsFlush += destRect;
             repaintManager->markNeedsFlush(pw, needsFlush, toplevelOffset);
@@ -547,12 +506,13 @@ void QWidgetPrivate::scrollRect(const QRect &rect, int dx, int dy)
 
     static const bool accelEnv = qEnvironmentVariableIntValue("QT_NO_FAST_SCROLL") == 0;
 
-    const QRect clipR = clipRect();
-    const QRect scrollRect = rect & clipR;
-    const bool accelerateScroll = accelEnv && isOpaque && !q_func()->testAttribute(Qt::WA_WState_InPaintEvent);
+    QRect scrollRect = rect & clipRect();
+    bool overlapped = false;
+    bool accelerateScroll = accelEnv && isOpaque && !q_func()->testAttribute(Qt::WA_WState_InPaintEvent)
+                            && !(overlapped = isOverlapped(scrollRect.translated(data.crect.topLeft())));
 
     if (!accelerateScroll) {
-        if (!overlappedRegion(scrollRect.translated(data.crect.topLeft()), true).isEmpty()) {
+        if (overlapped) {
             QRegion region(scrollRect);
             subtractOpaqueSiblings(region);
             invalidateBackingStore(region);
@@ -564,23 +524,12 @@ void QWidgetPrivate::scrollRect(const QRect &rect, int dx, int dy)
         const QRect destRect = scrollRect.translated(dx, dy) & scrollRect;
         const QRect sourceRect = destRect.translated(-dx, -dy);
 
-        const QRegion overlappedExpose = (overlappedRegion(scrollRect.translated(data.crect.topLeft())))
-                .translated(-data.crect.topLeft()) & clipR;
         QRegion childExpose(scrollRect);
-
-        const qreal factor = QHighDpiScaling::factor(q->windowHandle());
-        if (overlappedExpose.isEmpty() || qFloor(factor) == factor) {
-            const QList<QRect> rectsToScroll =
-                    getSortedRectsToScroll(QRegion(sourceRect) - overlappedExpose, dx, dy);
-            for (const QRect &r : rectsToScroll) {
-                if (repaintManager->bltRect(r, dx, dy, q)) {
-                    childExpose -= r.translated(dx, dy);
-                }
-            }
+        if (sourceRect.isValid()) {
+            if (repaintManager->bltRect(sourceRect, dx, dy, q))
+                childExpose -= destRect;
         }
 
-        childExpose -= overlappedExpose;
-
         if (inDirtyList) {
             if (rect == q->rect()) {
                 dirty.translate(dx, dy);
@@ -597,8 +546,6 @@ void QWidgetPrivate::scrollRect(const QRect &rect, int dx, int dy)
         if (!q->updatesEnabled())
             return;
 
-        if (!overlappedExpose.isEmpty())
-            invalidateBackingStore(overlappedExpose);
         if (!childExpose.isEmpty()) {
             repaintManager->markDirty(childExpose, q);
             isScrolled = true;