QQmlDelegateModelPrivate::itemsRemoved - keep persisted items in cache

QQmlDelegateModelPrivate::itemsRemoved removes unreferenced items from
the cache. If an item is persisted and unreferenced, it's disappearance
from the cache will prevent display models (e.g. qqmllistmodel) from
looping over all delegates. This can lead incomplete clear operations
and dangling screen artifacts.

This patch adds a condition to prevent persited items from being
removed from the cache.

It adds a test function in tst_QQmlDelegateModel.

Fixes: QTBUG-98365
Change-Id: I52e6344be85ca64eadbf44378de32cde126ff07a
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit da38eaf433)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/qmlmodels/qqmldelegatemodel.cpp

@@ -1627,7 +1627,7 @@ void QQmlDelegateModelPrivate::itemsRemoved(
                         emitDestroyingItem(object);
                     cacheItem->scriptRef -= 1;
                 }
-                if (!cacheItem->isReferenced()) {
+                if (!cacheItem->isReferenced() && !remove.inGroup(Compositor::Persisted)) {
                     m_compositor.clearFlags(Compositor::Cache, cacheIndex, 1, Compositor::CacheFlag);
                     m_cache.removeAt(cacheIndex);
                     delete cacheItem;

tests/auto/qml/qqmldelegatemodel/data/persistedItemsCache.qml

@@ -0,0 +1,62 @@
+import QtQuick
+import QtQuick.Window
+import QtQml.Models
+
+Window {
+    id: win
+    visible: true
+    width: 640
+    height: 480
+    property int destroyCount : 0;
+    property int createCount : 0;
+    property alias testListModel: mdl
+
+    DelegateModel {
+        id: visualModel
+        model: ListModel {
+            id: mdl
+            ListElement {
+                name: "a"
+                hidden: false
+            }
+            ListElement {
+                name: "b"
+                hidden: true
+            }
+            ListElement {
+                name: "c"
+                hidden: false
+            }
+        }
+
+        filterOnGroup: "selected"
+
+        groups: [
+            DelegateModelGroup {
+                name: "selected"
+                includeByDefault: true
+            }
+        ]
+
+        delegate: Text {
+            visible: DelegateModel.inSelected
+            property var idx
+            Component.onCompleted: {
+                ++createCount
+                idx = index
+                DelegateModel.inPersistedItems = true
+                DelegateModel.inSelected = !model.hidden
+            }
+            Component.onDestruction: ++destroyCount
+            text: model.name
+        }
+    }
+
+    ListView {
+    id: listView
+        model: visualModel
+        anchors.fill: parent
+        focus: true
+    }
+
+}

tests/auto/qml/qqmldelegatemodel/tst_qqmldelegatemodel.cpp

@@ -7,6 +7,7 @@
 #include <QtQml/qqmlcomponent.h>
 #include <QtQml/qqmlapplicationengine.h>
 #include <QtQmlModels/private/qqmldelegatemodel_p.h>
+#include <QtQmlModels/private/qqmllistmodel_p.h>
 #include <QtQuick/qquickview.h>
 #include <QtQuick/qquickitem.h>
 #include <QtQuickTestUtils/private/qmlutils_p.h>
@@ -31,6 +32,7 @@ private slots:
     void universalModelData();
     void typedModelData();
     void deleteRace();
+    void persistedItemsStayInCache();
 };
 
 class AbstractItemModel : public QAbstractItemModel
@@ -457,6 +459,21 @@ void tst_QQmlDelegateModel::deleteRace()
     QTRY_COMPARE(o->property("count").toInt(), 0);
 }
 
+void tst_QQmlDelegateModel::persistedItemsStayInCache()
+{
+    QQmlEngine engine;
+    QQmlComponent component(&engine, testFileUrl("persistedItemsCache.qml"));
+    QVERIFY2(component.isReady(), qPrintable(component.errorString()));
+    std::unique_ptr<QObject> object(component.create());
+    QVERIFY(object);
+    const QVariant properyListModel = object->property("testListModel");
+    QQmlListModel *listModel = qvariant_cast<QQmlListModel *>(properyListModel);
+    QVERIFY(listModel);
+    QTRY_COMPARE(object->property("createCount").toInt(), 3);
+    listModel->clear();
+    QTRY_COMPARE(object->property("destroyCount").toInt(), 3);
+}
+
 QTEST_MAIN(tst_QQmlDelegateModel)
 
 #include "tst_qqmldelegatemodel.moc"