QListView: fix AdjustToContents (sizeAdjustPolicy)

Unlike an acceptable effect in QTableView + QTreeView
setAdjustPolicy(QAbstractScrollArea::AdjustToContents)
unfortunately didn't work for QListViews (and QListWidget).

This patch corrects QListViews AdjustToContents
behavior.

[ChangeLog][QtWidgets][QListView] A more correct implementation
of QListView::viewportSizeHint has been made. That
implies that setting the sizeAdjustPolicy to AdjustToContent
on QListView and QListWidget will now cause the view to
size after the contents and avoid scrollbars.

Pick-to: 6.2
Task-number: QTBUG-58749
Change-Id: I1675115f2348e2fcf0b2c39b451ef337e10eb872
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/itemviews/qlistview.cpp

@@ -3487,7 +3487,47 @@ int QListView::visualIndex(const QModelIndex &index) const
 */
 QSize QListView::viewportSizeHint() const
 {
-    return QAbstractItemView::viewportSizeHint();
+    Q_D(const QListView);
+    // We don't have a nice simple size hint for invalid or wrapping list views.
+    if (!d->model)
+        return QAbstractItemView::viewportSizeHint();
+    const int rc = d->model->rowCount();
+    if (rc == 0 || isWrapping())
+        return QAbstractItemView::viewportSizeHint();
+
+    QStyleOptionViewItem option;
+    initViewItemOption(&option);
+
+    if (uniformItemSizes()) {
+        QSize sz = d->cachedItemSize;
+        if (!sz.isValid()) {
+            QModelIndex idx = d->model->index(0, d->column, d->root);
+            sz = d->itemSize(option, idx);
+        }
+        sz.setHeight(rc * sz.height());
+        return sz;
+    }
+
+    // Using AdjustToContents with a high number of rows will normally not make sense, so we limit
+    // this to default 1000 (that is btw the default for QHeaderView::resizeContentsPrecision())
+    // (By setting the property _q_resizeContentPrecision the user can however override this).
+    int maximumRows = 1000;
+    const QVariant userOverrideValue = property("_q_resizeContentPrecision");
+    if (userOverrideValue.isValid() && userOverrideValue.toInt() > 0) {
+      maximumRows = userOverrideValue.toInt();
+    }
+    const int rowCount = qMin(rc, maximumRows);
+
+    int h = 0;
+    int w = 0;
+
+    for (int row = 0; row < rowCount; ++row) {
+        QModelIndex idx = d->model->index(row, d->column, d->root);
+        QSize itemSize = d->itemSize(option, idx);
+        h += itemSize.height();
+        w = qMax(w, itemSize.width());
+    }
+    return QSize(w, h);
 }
 
 QT_END_NAMESPACE

tests/auto/widgets/itemviews/qlistview/tst_qlistview.cpp

@@ -38,6 +38,8 @@
 #include <QTimer>
 #include <QtMath>
 #include <QProxyStyle>
+#include <QVBoxLayout>
+#include <QDialog>
 
 #include <QtTest/private/qtesthelpers_p.h>
 #include <QtWidgets/private/qlistview_p.h>
@@ -166,6 +168,7 @@ private slots:
     void taskQTBUG_39902_mutualScrollBars();
     void horizontalScrollingByVerticalWheelEvents();
     void taskQTBUG_7232_AllowUserToControlSingleStep();
+    void taskQTBUG_58749_adjustToContent();
     void taskQTBUG_51086_skippingIndexesInSelectedIndexes();
     void taskQTBUG_47694_indexOutOfBoundBatchLayout();
     void itemAlignment();
@@ -2527,6 +2530,45 @@ void tst_QListView::taskQTBUG_7232_AllowUserToControlSingleStep()
     QCOMPARE(hStep1, lv.horizontalScrollBar()->singleStep());
 }
 
+void tst_QListView::taskQTBUG_58749_adjustToContent()
+{
+    QStandardItemModel model;
+    model.setRowCount(20);
+    model.setColumnCount(1);
+    const QString rowStr = QStringLiteral("Row number txt:");
+    for (int u = 0; u < model.rowCount(); ++u)
+      model.setData(model.index(u, 0), rowStr + QString::number(u));
+
+    QDialog w; // It really should work for QWidget, too, but sometimes an event (like move)
+               // is needed to get the resize triggered.
+    QVBoxLayout *l = new QVBoxLayout(&w);
+    l->setSizeConstraint(QLayout::SetFixedSize);
+    auto *view = new QListView;
+    view->setModel(&model);
+    view->setSizeAdjustPolicy(QAbstractScrollArea::AdjustToContents);
+    l->addWidget(view);
+    l->setSizeConstraint(QLayout::SetFixedSize);
+    w.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&w));
+
+    const QString longText = "Here we have a row text that is somewhat longer ...";
+
+    QFontMetrics fm(w.font(), &w);
+    QRect r = fm.boundingRect(model.data(model.index(0, 0)).toString());
+    const int longTextWidth = fm.horizontalAdvance(longText);
+    QVERIFY(w.height() > r.height() * model.rowCount());
+    // We have a width longer than the width for the given index data.
+    QVERIFY(w.width() > r.width());
+    // but ... the width should not have a width matching the much longer text.
+    QVERIFY(w.width() < longTextWidth);
+
+    // use the long text and make sure the width is adjusted.
+    model.setData(model.index(0, 0), longText);
+    QApplication::processEvents();
+    QVERIFY(w.width() > longTextWidth);
+    QVERIFY(view->width() >= longTextWidth);
+}
+
 void tst_QListView::taskQTBUG_51086_skippingIndexesInSelectedIndexes()
 {
     QStandardItemModel data(10, 1);