Make sure we paint overlapped children and siblings when moving far

In paintAndFlush, QWidgetRepaintManager subtracts opaque children if
the target isn't overlapped and isMoved is set to true. So in moveRect,
set isMoved to true after the blitting of movable areas, and reset it to
false if we have overlapped sibling or child regions. Otherwise, moving
so far that sourceRect is invalid (none of the original pixels are
visible after the move) we end up in a code path that sets isMoved to
true even with overlapping children or siblings, which then breaks
paintAndFlush's assumptions.

Reuse the test case written by Sergiy Korobov <tiamatenko@gmail.com> in
earlier attempts to fix this bug.

Fixes: QTBUG-26269
Pick-to: 6.2
Change-Id: If7443863f5eee79a80220cd587522122f42a21e4
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 13 insertions, 11 deletions

diff --git a/src/widgets/kernel/qwidgetrepaintmanager.cpp b/src/widgets/kernel/qwidgetrepaintmanager.cpp
index 977a746c5a..d9375c7281 100644
--- a/src/widgets/kernel/qwidgetrepaintmanager.cpp
+++ b/src/widgets/kernel/qwidgetrepaintmanager.cpp
@@ -449,12 +449,8 @@ 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();
-    QRect clipR(pd->clipRect());
+    const 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);
 
@@ -476,9 +472,13 @@ 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(newRect & clipR);
+        QRegion childExpose = QRegion(newRect) & clipR;
         QRegion overlappedExpose;
 
         if (sourceRect.isValid()) {
@@ -493,6 +493,7 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
                         childExpose -= r.translated(dx, dy);
                     }
                 }
+                isMoved = true;
             }
 
             childExpose -= overlappedExpose;
@@ -503,14 +504,17 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
 
         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 = true;
+                isMoved = false;
             }
         }
 
@@ -519,12 +523,10 @@ 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) {
+        if (childUpdatesEnabled && sourceRect.isValid()) {
             QRegion needsFlush(sourceRect);
             needsFlush += destRect;
             repaintManager->markNeedsFlush(pw, needsFlush, toplevelOffset);