Performance issue with QGraphicsItem::ItemClipsChildrenToShape.

If the child rect is bigger than the parent rect and parent has the
ItemClipsChildrenToShape flag set, then by updating the child, the whole
child rect is marked as dirty, resulting in a much larger update area
than required. This has a major impact on performance in Orbit/HB,
where e.g. item-views typically consist of a container item that clips
its children/items to shape. See attached video in QTBUG-9024.

Auto test included.

Task-number: QTBUG-9024

1 files changed, 13 insertions, 1 deletions

diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index 36fd5c8..dfba7c9 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -5116,9 +5116,15 @@ void QGraphicsScenePrivate::processDirtyItemsRecursive(QGraphicsItem *item, bool
 
     // Process children.
     if (itemHasChildren && item->d_ptr->dirtyChildren) {
+        const bool itemClipsChildrenToShape = item->d_ptr->flags & QGraphicsItem::ItemClipsChildrenToShape;
+        if (itemClipsChildrenToShape) {
+            // Make sure child updates are clipped to the item's bounding rect.
+            for (int i = 0; i < views.size(); ++i)
+                views.at(i)->d_func()->setUpdateClip(item);
+        }
         if (!dirtyAncestorContainsChildren) {
             dirtyAncestorContainsChildren = item->d_ptr->fullUpdatePending
-                                            && (item->d_ptr->flags & QGraphicsItem::ItemClipsChildrenToShape);
+                                            && itemClipsChildrenToShape;
         }
         const bool allChildrenDirty = item->d_ptr->allChildrenDirty;
         const bool parentIgnoresVisible = item->d_ptr->ignoreVisible;
@@ -5141,6 +5147,12 @@ void QGraphicsScenePrivate::processDirtyItemsRecursive(QGraphicsItem *item, bool
             }
             processDirtyItemsRecursive(child, dirtyAncestorContainsChildren, opacity);
         }
+
+        if (itemClipsChildrenToShape) {
+            // Reset updateClip.
+            for (int i = 0; i < views.size(); ++i)
+                views.at(i)->d_func()->setUpdateClip(0);
+        }
     } else if (wasDirtyParentSceneTransform) {
         item->d_ptr->invalidateChildrenSceneTransform();
     }