widgets: Mark widgets as needing flush during painting

Except for the case of syncing in response to an expose event,
where the platform asked us to flush a specific region, we
should strive to only flush parts that have been repainted.

And we should flush those parts to their nearest native child,
instead of unconditionally flushing the root/top level widget
as well.

By allowing drawWidget to schedule the flush we automatically
flush the minimal region, to the right widgets.

(manually cherry-picked from commit 79bf1b7e348d186934b14c417859a48bf9b3a06c)

Task-number: QTBUG-77447
Change-Id: I1a8fa703c0b7172655271e2a8f5eca694cb14129
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

2 files changed, 4 insertions, 12 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index bf339ca5c5..f5cc6bb853 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -5520,9 +5520,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
             }
             sourced->context = 0;
 
-            // Native widgets need to be marked dirty on screen so painting will be done in correct context
-            // Same check as in the no effects case below.
-            if (backingStore && !onScreen && !asRoot && (q->internalWinId() || !q->nativeParentWidget()->isWindow()))
+            if (backingStore)
                 backingStore->markDirtyOnScreen(rgn, q, offset);
 
             return;
@@ -5641,8 +5639,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 sendPaintEvent(toBePainted);
             }
 
-            // Native widgets need to be marked dirty on screen so painting will be done in correct context
-            if (backingStore && !onScreen && !asRoot && (q->internalWinId() || (q->nativeParentWidget() && !q->nativeParentWidget()->isWindow())))
+            if (backingStore)
                 backingStore->markDirtyOnScreen(toBePainted, q, offset);
 
             //restore
diff --git a/src/widgets/kernel/qwidgetbackingstore.cpp b/src/widgets/kernel/qwidgetbackingstore.cpp
index 24b8665013..b9505a6ed0 100644
--- a/src/widgets/kernel/qwidgetbackingstore.cpp
+++ b/src/widgets/kernel/qwidgetbackingstore.cpp
@@ -333,9 +333,6 @@ void QWidgetBackingStore::beginPaint(QRegion &toClean, QWidget *widget, QBacking
     Q_UNUSED(widget);
     Q_UNUSED(toCleanIsInTopLevelCoordinates);
 
-    // Always flush repainted areas.
-    dirtyOnScreen += toClean;
-
 #ifdef QT_NO_PAINT_DEBUG
     backingStore->beginPaint(toClean);
 #else
@@ -714,8 +711,7 @@ void QWidgetBackingStore::markDirtyOnScreen(const QRegion &region, QWidget *widg
 
     // Top-level.
     if (widget == tlw) {
-        if (!widget->testAttribute(Qt::WA_WState_InPaintEvent))
-            dirtyOnScreen += region;
+        dirtyOnScreen += region;
         return;
     }
 
@@ -723,8 +719,7 @@ void QWidgetBackingStore::markDirtyOnScreen(const QRegion &region, QWidget *widg
     if (!hasPlatformWindow(widget) && !widget->isWindow()) {
         QWidget *nativeParent = widget->nativeParentWidget();        // Alien widgets with the top-level as the native parent (common case).
         if (nativeParent == tlw) {
-            if (!widget->testAttribute(Qt::WA_WState_InPaintEvent))
-                dirtyOnScreen += region.translated(topLevelOffset);
+            dirtyOnScreen += region.translated(topLevelOffset);
             return;
         }