QListView: Catch stack overflow on mutual scrollbar calculation.

Task-number: QTBUG-39902
Change-Id: Ie850371098070e8ce485d5cb122aa89c18d97359
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

2 files changed, 57 insertions, 2 deletions

diff --git a/src/widgets/itemviews/qlistview.cpp b/src/widgets/itemviews/qlistview.cpp
index 135f89d4ac..5706be4b6d 100644
--- a/src/widgets/itemviews/qlistview.cpp
+++ b/src/widgets/itemviews/qlistview.cpp
@@ -1846,14 +1846,44 @@ void QCommonListViewBase::updateHorizontalScrollBar(const QSize &step)
 {
     horizontalScrollBar()->setSingleStep(step.width() + spacing());
     horizontalScrollBar()->setPageStep(viewport()->width());
-    horizontalScrollBar()->setRange(0, contentsSize.width() - viewport()->width());
+
+    // If both scroll bars are set to auto, we might end up in a situation with enough space
+    // for the actual content. But still one of the scroll bars will become enabled due to
+    // the other one using the space. The other one will become invisible in the same cycle.
+    // -> Infinite loop, QTBUG-39902
+    const bool bothScrollBarsAuto = qq->verticalScrollBarPolicy() == Qt::ScrollBarAsNeeded &&
+                                    qq->horizontalScrollBarPolicy() == Qt::ScrollBarAsNeeded;
+
+    if (bothScrollBarsAuto && contentsSize.width() - qq->verticalScrollBar()->width() <= viewport()->width()
+                           && contentsSize.height() - qq->horizontalScrollBar()->height() <= viewport()->height()) {
+        // break the infinite loop described above by setting the range to 0, 0.
+        // QAbstractScrollArea will then hide the scroll bar for us
+        horizontalScrollBar()->setRange(0, 0);
+    } else {
+        horizontalScrollBar()->setRange(0, contentsSize.width() - viewport()->width());
+    }
 }
 
 void QCommonListViewBase::updateVerticalScrollBar(const QSize &step)
 {
     verticalScrollBar()->setSingleStep(step.height() + spacing());
     verticalScrollBar()->setPageStep(viewport()->height());
-    verticalScrollBar()->setRange(0, contentsSize.height() - viewport()->height());
+
+    // If both scroll bars are set to auto, we might end up in a situation with enough space
+    // for the actual content. But still one of the scroll bars will become enabled due to
+    // the other one using the space. The other one will become invisible in the same cycle.
+    // -> Infinite loop, QTBUG-39902
+    const bool bothScrollBarsAuto = qq->verticalScrollBarPolicy() == Qt::ScrollBarAsNeeded &&
+                                    qq->horizontalScrollBarPolicy() == Qt::ScrollBarAsNeeded;
+
+    if (bothScrollBarsAuto && contentsSize.width() - qq->verticalScrollBar()->width() <= viewport()->width()
+                           && contentsSize.height() - qq->horizontalScrollBar()->height() <= viewport()->height()) {
+        // break the infinite loop described above by setting the range to 0, 0.
+        // QAbstractScrollArea will then hide the scroll bar for us
+        verticalScrollBar()->setRange(0, 0);
+    } else {
+        verticalScrollBar()->setRange(0, contentsSize.height() - viewport()->height());
+    }
 }
 
 void QCommonListViewBase::scrollContentsBy(int dx, int dy, bool /*scrollElasticBand*/)
diff --git a/tests/auto/widgets/itemviews/qlistview/tst_qlistview.cpp b/tests/auto/widgets/itemviews/qlistview/tst_qlistview.cpp
index b36b5aef8a..bb05db0b15 100644
--- a/tests/auto/widgets/itemviews/qlistview/tst_qlistview.cpp
+++ b/tests/auto/widgets/itemviews/qlistview/tst_qlistview.cpp
@@ -155,6 +155,7 @@ private slots:
     void spacing();
     void testScrollToWithHidden();
     void testViewOptions();
+    void taskQTBUG_39902_mutualScrollBars();
 };
 
 // Testing get/set functions
@@ -2355,5 +2356,29 @@ void tst_QListView::testViewOptions()
     QCOMPARE(options.decorationPosition, QStyleOptionViewItem::Top);
 }
 
+void tst_QListView::taskQTBUG_39902_mutualScrollBars()
+{
+    QWidget window;
+    window.resize(400, 300);
+    QListView *view = new QListView(&window);
+    QStandardItemModel model(200, 1);
+    const QSize itemSize(100, 20);
+    for (int i = 0; i < model.rowCount(); ++i)
+        model.setData(model.index(i, 0), itemSize, Qt::SizeHintRole);
+    view->setModel(&model);
+
+    window.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&window));
+    // make sure QListView is done with layouting the items (1/10 sec, like QListView)
+    QTest::qWait(100);
+
+    model.setRowCount(2);
+    for (int i = 0; i < model.rowCount(); ++i)
+        model.setData(model.index(i, 0), itemSize, Qt::SizeHintRole);
+    view->resize(itemSize.width() + view->frameWidth() * 2, model.rowCount() * itemSize.height() + view->frameWidth() * 2);
+    // this will end up in a stack overflow, if QTBUG-39902 is not fixed
+    QTest::qWait(100);
+}
+
 QTEST_MAIN(tst_QListView)
 #include "tst_qlistview.moc"