GridView: Fix an off-by-one offset of positioning of items

The block for finding the position for the item *before* needs to be
applied for the case where visibleItems is empty too, so we separate it
out.

Change-Id: I60a7e3a44d01a0087970e37bf4e73b94da5ebd48
Task-number: QTBUG-49218
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 157 insertions, 0 deletions

diff --git a/tests/auto/quick/qquickgridview/data/qtbug49218.qml b/tests/auto/quick/qquickgridview/data/qtbug49218.qml
new file mode 100644
index 0000000000..1a55fe5b3a
--- /dev/null
+++ b/tests/auto/quick/qquickgridview/data/qtbug49218.qml
@@ -0,0 +1,126 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** BSD License Usage
+** Alternatively, you may use this file under the terms of the BSD license
+** as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of The Qt Company Ltd nor the names of its
+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+import QtQuick 2.4
+
+Item {
+    width: 500
+    height: 160
+    visible: true
+
+    property var model1: ["1","2","3","4","5","6","7","8","9","10",
+    "11","12","13","14","15","16","17","18","19","20","21","22","23","24","25","26","27","28","29","30",
+    "31","32","33","34","a"]
+    property var model2: ["a","b","c","d","e","f","g","h","i","j","k","l","m","1"]
+    property bool useModel1: true
+
+    function changeModel() {
+        useModel1 = !useModel1
+        grid.loadModel(useModel1 ? model1 : model2)
+    }
+
+    function scrollToTop() {
+        grid.contentY = grid.originY;
+    }
+
+    GridView {
+        id: grid
+        anchors.fill: parent
+
+        model: ListModel {
+        }
+
+        onCurrentIndexChanged: {
+            positionViewAtIndex(currentIndex, GridView.Contain)
+        }
+
+        Component.onCompleted: {
+            loadModel(model1)
+            grid.currentIndex = 34
+            grid.positionViewAtIndex(34, GridView.Contain)
+        }
+
+        function loadModel(m) {
+            var remove = {};
+            var add = {};
+            var i;
+            for (i=0; i < model.count; ++i)
+                remove[model.get(i).name] = true;
+            for (i=0; i < m.length; ++i)
+                if (remove[m[i]])
+                    delete remove[m[i]];
+                else
+                    add[m[i]] = true;
+
+            for (i=model.count-1; i>= 0; --i)
+                if (remove[model.get(i).name])
+                    model.remove(i, 1);
+
+            for (i=0; i<m.length; ++i)
+                if (add[m[i]])
+                    model.insert(i, { "name": m[i] })
+        }
+
+        delegate: Rectangle {
+            height: grid.cellHeight
+            width: grid.cellWidth
+            color: GridView.isCurrentItem ? "gray" : "white"
+            Text {
+                anchors.fill: parent
+                text: name
+            }
+            MouseArea {
+                anchors.fill: parent
+                onClicked: {
+                    grid.currentIndex = index
+                }
+            }
+        }
+    }
+}
diff --git a/tests/auto/quick/qquickgridview/tst_qquickgridview.cpp b/tests/auto/quick/qquickgridview/tst_qquickgridview.cpp
index 388ecc2ab8..e271cc1d7b 100644
--- a/tests/auto/quick/qquickgridview/tst_qquickgridview.cpp
+++ b/tests/auto/quick/qquickgridview/tst_qquickgridview.cpp
@@ -210,6 +210,7 @@ private slots:
     void contentHeightWithDelayRemove();
 
     void QTBUG_45640();
+    void QTBUG_49218();
     void QTBUG_48870_fastModelUpdates();
 
     void keyNavigationEnabled();
@@ -6629,6 +6630,36 @@ void tst_QQuickGridView::QTBUG_45640()
     delete window;
 }
 
+void tst_QQuickGridView::QTBUG_49218()
+{
+    QQuickView *window = createView();
+    window->setSource(testFileUrl("qtbug49218.qml"));
+    window->show();
+    QVERIFY(QTest::qWaitForWindowExposed(window));
+
+    QQuickItem *rootItem = qobject_cast<QQuickItem*>(window->rootObject());
+    QQuickGridView *gridview = qobject_cast<QQuickGridView *>(rootItem->childItems().first());
+    QVERIFY(gridview != 0);
+
+    auto processEventsAndForceLayout = [&gridview] () {
+        for (int pass = 0; pass < 2; ++pass) {
+            QCoreApplication::sendPostedEvents(nullptr, QEvent::DeferredDelete);
+            gridview->forceLayout();
+        }
+    };
+    QMetaObject::invokeMethod(rootItem, "scrollToTop");
+    processEventsAndForceLayout();
+    QMetaObject::invokeMethod(rootItem, "changeModel");
+    processEventsAndForceLayout();
+    QMetaObject::invokeMethod(rootItem, "changeModel");
+    processEventsAndForceLayout();
+    QMetaObject::invokeMethod(rootItem, "scrollToTop");
+    processEventsAndForceLayout();
+
+    QCOMPARE(gridview->indexAt(gridview->cellWidth() - 10, gridview->cellHeight() - 10), 0);
+    delete window;
+}
+
 void tst_QQuickGridView::keyNavigationEnabled()
 {
     QScopedPointer<QQuickView> window(createView());