QQuickFlickable: fix division by zero

The issue could be seen when enabling exceptions and running the
following QML code:

    Flickable {
        id: flickable
        anchors.fill: parent
        contentWidth: 1000
        contentHeight: 1000

        Text {
            text: flickable.visibleArea.xPosition
        }
    }

Change-Id: I615f9f9dc84903fb3a902f416a55e3ce3fece64c
Fixes: QTBUG-81098
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

3 files changed, 64 insertions, 4 deletions

diff --git a/src/quick/items/qquickflickable.cpp b/src/quick/items/qquickflickable.cpp
index d9ec7de611..52f7c29aa4 100644
--- a/src/quick/items/qquickflickable.cpp
+++ b/src/quick/items/qquickflickable.cpp
@@ -128,8 +128,13 @@ void QQuickFlickableVisibleArea::updateVisible()
     // Vertical
     const qreal viewheight = flickable->height();
     const qreal maxyextent = -flickable->maxYExtent() + flickable->minYExtent();
-    qreal pagePos = (-p->vData.move.value() + flickable->minYExtent()) / (maxyextent + viewheight);
-    qreal pageSize = viewheight / (maxyextent + viewheight);
+    const qreal maxYBounds = maxyextent + viewheight;
+    qreal pagePos = 0;
+    qreal pageSize = 0;
+    if (!qFuzzyIsNull(maxYBounds)) {
+        pagePos = (-p->vData.move.value() + flickable->minYExtent()) / maxYBounds;
+        pageSize = viewheight / maxYBounds;
+    }
 
     if (pageSize != m_heightRatio) {
         m_heightRatio = pageSize;
@@ -143,8 +148,14 @@ void QQuickFlickableVisibleArea::updateVisible()
     // Horizontal
     const qreal viewwidth = flickable->width();
     const qreal maxxextent = -flickable->maxXExtent() + flickable->minXExtent();
-    pagePos = (-p->hData.move.value() + flickable->minXExtent()) / (maxxextent + viewwidth);
-    pageSize = viewwidth / (maxxextent + viewwidth);
+    const qreal maxXBounds = maxxextent + viewwidth;
+    if (!qFuzzyIsNull(maxXBounds)) {
+        pagePos = (-p->hData.move.value() + flickable->minXExtent()) / maxXBounds;
+        pageSize = viewwidth / maxXBounds;
+    } else {
+        pagePos = 0;
+        pageSize = 0;
+    }
 
     if (pageSize != m_widthRatio) {
         m_widthRatio = pageSize;
diff --git a/tests/auto/quick/qquickflickable/data/visibleAreaBinding.qml b/tests/auto/quick/qquickflickable/data/visibleAreaBinding.qml
new file mode 100644
index 0000000000..6f7bc89793
--- /dev/null
+++ b/tests/auto/quick/qquickflickable/data/visibleAreaBinding.qml
@@ -0,0 +1,38 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** 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.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick 2.14
+
+Flickable {
+    id: flickable
+    // Deliberately has no size set.
+
+    Text {
+        text: flickable.visibleArea.xPosition
+    }
+}
diff --git a/tests/auto/quick/qquickflickable/tst_qquickflickable.cpp b/tests/auto/quick/qquickflickable/tst_qquickflickable.cpp
index c104eecbcd..da4bf5dae3 100644
--- a/tests/auto/quick/qquickflickable/tst_qquickflickable.cpp
+++ b/tests/auto/quick/qquickflickable/tst_qquickflickable.cpp
@@ -205,6 +205,7 @@ private slots:
     void overshoot_reentrant();
     void synchronousDrag_data();
     void synchronousDrag();
+    void visibleAreaBinding();
 
 private:
     void flickWithTouch(QQuickWindow *window, const QPoint &from, const QPoint &to);
@@ -2541,6 +2542,16 @@ void tst_qquickflickable::synchronousDrag()
     QTest::touchEvent(window, touchDevice).release(0, p5, window);
 }
 
+// QTBUG-81098: tests that a binding to visibleArea doesn't result
+// in a division-by-zero exception (when exceptions are enabled).
+void tst_qquickflickable::visibleAreaBinding()
+{
+    QScopedPointer<QQuickView> window(new QQuickView);
+    window->setSource(testFileUrl("visibleAreaBinding.qml"));
+    QTRY_COMPARE(window->status(), QQuickView::Ready);
+    // Shouldn't crash.
+}
+
 QTEST_MAIN(tst_qquickflickable)
 
 #include "tst_qquickflickable.moc"