Fix performance issue with drag retrigger events for qtquick items

The items that reject drag events (specifically onEntered in DropArea)
should not be retriggered with DragEnter event until entered once again.

Fixes: QTBUG-74496
Change-Id: I241a6004da6382685be89fe8a001b98dfde5c8a2
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
(cherry picked from commit 0b7374fefa1abf08e956bc5d34008352144bd9ae)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

4 files changed, 72 insertions, 11 deletions

diff --git a/src/quick/items/qquickdrag_p.h b/src/quick/items/qquickdrag_p.h
index af466836d9..58d94738eb 100644
--- a/src/quick/items/qquickdrag_p.h
+++ b/src/quick/items/qquickdrag_p.h
@@ -109,9 +109,12 @@ public:
     void grab(QQuickItem *item) { m_items.insert(new Item(item)); }
     iterator release(iterator at) { Item *item = *at; at = at.erase(); delete item; return at; }
 
+    auto& ignoreList() { return m_ignoreDragItems; }
+
 private:
 
     ItemList m_items;
+    QVarLengthArray<QQuickItem *, 4> m_ignoreDragItems;
     QObject *m_target;
 };
 
diff --git a/src/quick/util/qquickdeliveryagent.cpp b/src/quick/util/qquickdeliveryagent.cpp
index 48b2d3ebd1..90c9f11dd9 100644
--- a/src/quick/util/qquickdeliveryagent.cpp
+++ b/src/quick/util/qquickdeliveryagent.cpp
@@ -2187,6 +2187,7 @@ void QQuickDeliveryAgentPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QE
             QDragLeaveEvent leaveEvent;
             for (; grabItem != grabber->end(); grabItem = grabber->release(grabItem))
                 QCoreApplication::sendEvent(**grabItem, &leaveEvent);
+            grabber->ignoreList().clear();
             return;
         } else {
             QDragMoveEvent *moveEvent = static_cast<QDragMoveEvent *>(event);
@@ -2244,6 +2245,8 @@ void QQuickDeliveryAgentPrivate::deliverDragEvent(QQuickDragGrabber *grabber, QE
                 e->modifiers());
         QQuickDropEventEx::copyActions(&enterEvent, *e);
         event->setAccepted(deliverDragEvent(grabber, rootItem, &enterEvent));
+    } else {
+        grabber->ignoreList().clear();
     }
 }
 
@@ -2257,8 +2260,13 @@ bool QQuickDeliveryAgentPrivate::deliverDragEvent(
     QPointF p = item->mapFromScene(event->position().toPoint());
     bool itemContained = item->contains(p);
 
-    if (!itemContained && itemPrivate->flags & QQuickItem::ItemClipsChildrenToShape) {
-        return false;
+    const int itemIndex = grabber->ignoreList().indexOf(item);
+    if (!itemContained) {
+        if (itemIndex >= 0)
+            grabber->ignoreList().remove(itemIndex);
+
+        if (itemPrivate->flags & QQuickItem::ItemClipsChildrenToShape)
+            return false;
     }
 
     QDragEnterEvent enterEvent(
@@ -2288,15 +2296,19 @@ bool QQuickDeliveryAgentPrivate::deliverDragEvent(
         }
 
         if (event->type() == QEvent::DragMove || itemPrivate->flags & QQuickItem::ItemAcceptsDrops) {
-            if (event->type() == QEvent::DragEnter && formerTarget) {
-                QQuickItem *formerTargetItem = qobject_cast<QQuickItem *>(formerTarget);
-                if (formerTargetItem && currentGrabItems) {
-                    QDragLeaveEvent leaveEvent;
-                    QCoreApplication::sendEvent(formerTarget, &leaveEvent);
-
-                    // Remove the item from the currentGrabItems so a leave event won't be generated
-                    // later on
-                    currentGrabItems->removeAll(formerTarget);
+            if (event->type() == QEvent::DragEnter) {
+                if (formerTarget) {
+                    QQuickItem *formerTargetItem = qobject_cast<QQuickItem *>(formerTarget);
+                    if (formerTargetItem && currentGrabItems) {
+                        QDragLeaveEvent leaveEvent;
+                        QCoreApplication::sendEvent(formerTarget, &leaveEvent);
+
+                        // Remove the item from the currentGrabItems so a leave event won't be generated
+                        // later on
+                        currentGrabItems->removeAll(formerTarget);
+                    }
+                } else if (itemIndex >= 0) {
+                    return false;
                 }
             }
 
@@ -2312,6 +2324,8 @@ bool QQuickDeliveryAgentPrivate::deliverDragEvent(
                     grabber->grab(item);
                     grabber->setTarget(item);
                     return true;
+                } else if (itemIndex < 0) {
+                    grabber->ignoreList().append(item);
                 }
             } else {
                 return true;
diff --git a/tests/auto/quick/qquickdroparea/data/ignoreRetriggerEvent.qml b/tests/auto/quick/qquickdroparea/data/ignoreRetriggerEvent.qml
new file mode 100644
index 0000000000..af25a04ee7
--- /dev/null
+++ b/tests/auto/quick/qquickdroparea/data/ignoreRetriggerEvent.qml
@@ -0,0 +1,15 @@
+import QtQuick 2.0
+
+DropArea {
+    property int enterEvents: 0
+    property int exitEvents: 0
+    width: 100; height: 100
+    objectName: "dropArea"
+    onEntered: function (drag) { ++enterEvents; drag.accepted = false }
+    onExited: {++exitEvents}
+        Item {
+        objectName: "dragItem"
+        x: 50; y: 50
+        width: 10; height: 10
+    }
+}
diff --git a/tests/auto/quick/qquickdroparea/tst_qquickdroparea.cpp b/tests/auto/quick/qquickdroparea/tst_qquickdroparea.cpp
index 6bbe0d522b..9c9d059a52 100644
--- a/tests/auto/quick/qquickdroparea/tst_qquickdroparea.cpp
+++ b/tests/auto/quick/qquickdroparea/tst_qquickdroparea.cpp
@@ -67,6 +67,9 @@ public:
 private slots:
     void containsDrag_internal();
     void containsDrag_external();
+
+    void ignoreRetriggerEvent();
+
     void keys_internal();
     void keys_external();
     void source_internal();
@@ -829,6 +832,32 @@ void tst_QQuickDropArea::competingDrags()
     QCOMPARE(evaluate<QString>(dropArea1, "statuslol"), QStringLiteral("parent"));
 }
 
+void tst_QQuickDropArea::ignoreRetriggerEvent()
+{
+    QQuickView window;
+    QByteArray errorMessage;
+    QVERIFY2(QQuickTest::initView(window, testFileUrl("ignoreRetriggerEvent.qml"), true, &errorMessage), errorMessage.constData());
+
+    QQuickItem *dropArea = window.rootObject();
+    QVERIFY(dropArea);
+    QQuickItem *dragItem = dropArea->findChild<QQuickItem *>("dragItem");
+    QVERIFY(dragItem);
+
+    evaluate<void>(dragItem, "Drag.active = true");
+    // Drag the item within the drop area
+    dragItem->setPosition(QPointF(25, 25));
+    QCoreApplication::processEvents();
+    dragItem->setPosition(QPointF(50, 50));
+    QCoreApplication::processEvents();
+    dragItem->setPosition(QPointF(75, 75));
+    QCoreApplication::processEvents();
+
+    QCOMPARE(evaluate<bool>(dropArea, "containsDrag"), false);
+    QCOMPARE(evaluate<int>(dropArea, "enterEvents"), 1);
+    QCOMPARE(evaluate<int>(dropArea, "exitEvents"), 0);
+}
+
+
 void tst_QQuickDropArea::simultaneousDrags()
 {
     QQuickWindow window;