Crash fix in QTreeView::sizeHintForColumn().

Vertical scrollbar may get out of sync. When this happens, the calculation of
firstVisibleItem will retrun "-1". This must be handled in ::sizeHintForColumn().
Added an auto-test for the crashes.

Task-number: QTBUG-34717

Change-Id: I867fd144ef3ce45e382337c5eafe345f573cd944
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/itemviews/qtreeview.cpp

@@ -2858,6 +2858,21 @@ int QTreeView::sizeHintForColumn(int column) const
     int offset = 0;
     int start = d->firstVisibleItem(&offset);
     int end = d->lastVisibleItem(start, offset);
+    if (start < 0 || end < 0 || end == viewItems.size() - 1) {
+        end = viewItems.size() - 1;
+        if (maximumProcessRows < 0) {
+            start = 0;
+        } else if (maximumProcessRows == 0) {
+            start = qMax(0, end - 1);
+            int remainingHeight = viewport()->height();
+            while (start > 0 && remainingHeight > 0) {
+                remainingHeight -= d->itemHeight(start);
+                --start;
+            }
+        } else {
+            start = qMax(0, end - maximumProcessRows);
+        }
+    }
 
     int rowsProcessed = 0;
 
@@ -3606,8 +3621,11 @@ int QTreeViewPrivate::firstVisibleItem(int *offset) const
 
 int QTreeViewPrivate::lastVisibleItem(int firstVisual, int offset) const
 {
-    if (firstVisual < 0 || offset < 0)
+    if (firstVisual < 0 || offset < 0) {
         firstVisual = firstVisibleItem(&offset);
+        if (firstVisual < 0)
+            return -1;
+    }
     int y = - offset;
     int value = viewport->height();
 

tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp

@@ -259,6 +259,7 @@ private slots:
     void taskQTBUG_25333_adjustViewOptionsForIndex();
     void taskQTBUG_18539_emitLayoutChanged();
     void taskQTBUG_8176_emitOnExpandAll();
+    void taskQTBUG_34717_collapseAtBottom();
     void testInitialFocus();
 };
 
@@ -4240,6 +4241,35 @@ void tst_QTreeView::taskQTBUG_8176_emitOnExpandAll()
     QCOMPARE(spy2.size(), 1); // item2 is collapsed
 }
 
+// From QTBUG_34717 (QTreeWidget crashes when scrolling to the end
+// of an expanded tree, then collapse all)
+// The test passes simply if it doesn't crash.
+void tst_QTreeView::taskQTBUG_34717_collapseAtBottom()
+{
+    QTreeWidget treeWidget;
+    treeWidget.header()->setSectionResizeMode(QHeaderView::ResizeToContents);
+    treeWidget.setColumnCount(2);
+    QTreeWidgetItem *mainItem = new QTreeWidgetItem(&treeWidget, QStringList() << "Root");
+    for (int i = 0; i < 200; ++i) {
+        QTreeWidgetItem *item = new QTreeWidgetItem(mainItem, QStringList(QString("Item")));
+        new QTreeWidgetItem(item, QStringList() << "Child" << "1");
+        new QTreeWidgetItem(item, QStringList() << "Child" << "2");
+        new QTreeWidgetItem(item, QStringList() << "Child" << "3");
+    }
+    treeWidget.show();
+    treeWidget.expandAll();
+    treeWidget.scrollToBottom();
+    treeWidget.collapseAll();
+
+    treeWidget.setAnimated(true);
+    treeWidget.expandAll();
+    treeWidget.scrollToBottom();
+    mainItem->setExpanded(false);
+
+    PublicView *pview = (PublicView*) &treeWidget;
+    QVERIFY(pview->sizeHintForColumn(1) >= 0);
+}
+
 void tst_QTreeView::testInitialFocus()
 {
     QTreeWidget treeWidget;