QQuickWindow: Higher z-order items now steal drop target

Previously, an active drop target would remain the drop target until the
drag left it's area, or entered a child item that accepted a
DragEnterEvent, even if the drag entered a drop target with a globally
higher z-order from a different subtree.

When moving to an item with a higher z-order, the DragEnterEvent is
now sent to the new drop target before DragLeaveEvent is sent to the old
drop target.  There can now only be one drop target.  If an item is the
current drop target and a higher z-order child accepts the DragEnterEvent,
the parent is no longer a drop target.

Fixes: QTBUG-30305
Change-Id: I7b985d6317be70867e7727222a4cd44ace7559e6
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 67 insertions, 30 deletions

diff --git a/src/quick/items/qquickwindow.cpp b/src/quick/items/qquickwindow.cpp
index b83c9159a4..0bbc400d08 100644
--- a/src/quick/items/qquickwindow.cpp
+++ b/src/quick/items/qquickwindow.cpp
@@ -2828,32 +2828,49 @@ void QQuickWindowPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QEvent *e
             for (; grabItem != grabber->end(); grabItem = grabber->release(grabItem))
                 QCoreApplication::sendEvent(**grabItem, &leaveEvent);
             return;
-        } else for (; grabItem != grabber->end(); grabItem = grabber->release(grabItem)) {
+        } else {
             QDragMoveEvent *moveEvent = static_cast<QDragMoveEvent *>(event);
-            if (deliverDragEvent(grabber, **grabItem, moveEvent)) {
-                for (++grabItem; grabItem != grabber->end();) {
-                    QPointF p = (**grabItem)->mapFromScene(moveEvent->pos());
-                    if ((**grabItem)->contains(p)) {
-                        QDragMoveEvent translatedEvent(
-                                p.toPoint(),
-                                moveEvent->possibleActions(),
-                                moveEvent->mimeData(),
-                                moveEvent->mouseButtons(),
-                                moveEvent->keyboardModifiers());
-                        QQuickDropEventEx::copyActions(&translatedEvent, *moveEvent);
-                        QCoreApplication::sendEvent(**grabItem, &translatedEvent);
-                        ++grabItem;
-                    } else {
-                        QDragLeaveEvent leaveEvent;
-                        QCoreApplication::sendEvent(**grabItem, &leaveEvent);
-                        grabItem = grabber->release(grabItem);
-                    }
+
+            // Used to ensure we don't send DragEnterEvents to current drop targets,
+            // and to detect which current drop targets we have left
+            QVarLengthArray<QQuickItem*, 64> currentGrabItems;
+            for (; grabItem != grabber->end(); grabItem = grabber->release(grabItem))
+                currentGrabItems.append(**grabItem);
+
+            // Look for any other potential drop targets that are higher than the current ones
+            QDragEnterEvent enterEvent(
+                    moveEvent->pos(),
+                    moveEvent->possibleActions(),
+                    moveEvent->mimeData(),
+                    moveEvent->mouseButtons(),
+                    moveEvent->keyboardModifiers());
+            QQuickDropEventEx::copyActions(&enterEvent, *moveEvent);
+            event->setAccepted(deliverDragEvent(grabber, contentItem, &enterEvent, &currentGrabItems));
+
+            for (grabItem = grabber->begin(); grabItem != grabber->end(); ++grabItem) {
+                int i = currentGrabItems.indexOf(**grabItem);
+                if (i >= 0) {
+                    currentGrabItems.remove(i);
+                    // Still grabbed: send move event
+                    QDragMoveEvent translatedEvent(
+                            (**grabItem)->mapFromScene(moveEvent->pos()).toPoint(),
+                            moveEvent->possibleActions(),
+                            moveEvent->mimeData(),
+                            moveEvent->mouseButtons(),
+                            moveEvent->keyboardModifiers());
+                    QQuickDropEventEx::copyActions(&translatedEvent, *moveEvent);
+                    QCoreApplication::sendEvent(**grabItem, &translatedEvent);
+                    event->setAccepted(translatedEvent.isAccepted());
+                    QQuickDropEventEx::copyActions(moveEvent, translatedEvent);
                 }
-                return;
-            } else {
-                QDragLeaveEvent leaveEvent;
-                QCoreApplication::sendEvent(**grabItem, &leaveEvent);
             }
+
+            // Anything left in currentGrabItems is no longer a drop target and should be sent a DragLeaveEvent
+            QDragLeaveEvent leaveEvent;
+            for (QQuickItem *i : currentGrabItems)
+                QCoreApplication::sendEvent(i, &leaveEvent);
+
+            return;
         }
     }
     if (event->type() == QEvent::DragEnter || event->type() == QEvent::DragMove) {
@@ -2869,9 +2886,8 @@ void QQuickWindowPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QEvent *e
     }
 }
 
-bool QQuickWindowPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QQuickItem *item, QDragMoveEvent *event)
+bool QQuickWindowPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QQuickItem *item, QDragMoveEvent *event, QVarLengthArray<QQuickItem*, 64> *currentGrabItems)
 {
-    bool accepted = false;
     QQuickItemPrivate *itemPrivate = QQuickItemPrivate::get(item);
     if (!item->isVisible() || !item->isEnabled() || QQuickItemPrivate::get(item)->culled)
         return false;
@@ -2890,12 +2906,24 @@ bool QQuickWindowPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QQuickIte
             event->keyboardModifiers());
     QQuickDropEventEx::copyActions(&enterEvent, *event);
     QList<QQuickItem *> children = itemPrivate->paintOrderChildItems();
+
+    // Check children in front of this item first
     for (int ii = children.count() - 1; ii >= 0; --ii) {
-        if (deliverDragEvent(grabber, children.at(ii), &enterEvent))
+        if (children.at(ii)->z() < 0)
+            continue;
+        if (deliverDragEvent(grabber, children.at(ii), &enterEvent, currentGrabItems))
             return true;
     }
 
     if (itemContained) {
+        // If this item is currently grabbed, don't send it another DragEnter,
+        // just grab it again if it's still contained.
+        if (currentGrabItems && currentGrabItems->contains(item)) {
+            grabber->grab(item);
+            grabber->setTarget(item);
+            return true;
+        }
+
         if (event->type() == QEvent::DragMove || itemPrivate->flags & QQuickItem::ItemAcceptsDrops) {
             QDragMoveEvent translatedEvent(
                     p.toPoint(),
@@ -2912,15 +2940,24 @@ bool QQuickWindowPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QQuickIte
             if (event->type() == QEvent::DragEnter) {
                 if (translatedEvent.isAccepted()) {
                     grabber->grab(item);
-                    accepted = true;
+                    grabber->setTarget(item);
+                    return true;
                 }
             } else {
-                accepted = true;
+                return true;
             }
         }
     }
 
-    return accepted;
+    // Check children behind this item if this item or any higher children have not accepted
+    for (int ii = children.count() - 1; ii >= 0; --ii) {
+        if (children.at(ii)->z() >= 0)
+            continue;
+        if (deliverDragEvent(grabber, children.at(ii), &enterEvent, currentGrabItems))
+            return true;
+    }
+
+    return false;
 }
 #endif // quick_draganddrop
 
diff --git a/src/quick/items/qquickwindow_p.h b/src/quick/items/qquickwindow_p.h
index 7d1767c40e..165859b5f3 100644
--- a/src/quick/items/qquickwindow_p.h
+++ b/src/quick/items/qquickwindow_p.h
@@ -192,7 +192,7 @@ public:
 
 #if QT_CONFIG(quick_draganddrop)
     void deliverDragEvent(QQuickDragGrabber *, QEvent *);
-    bool deliverDragEvent(QQuickDragGrabber *, QQuickItem *, QDragMoveEvent *);
+    bool deliverDragEvent(QQuickDragGrabber *, QQuickItem *, QDragMoveEvent *, QVarLengthArray<QQuickItem*, 64> *currentGrabItems = nullptr);
 #endif
 #if QT_CONFIG(cursor)
     void updateCursor(const QPointF &scenePos);