QDialogButtonBox: Don't set focus in a dialog with StrongFocus children

A QDialogButtonBox with the first accept button becoming default, didn't
explicitly set focus on such a button in a QDialog.
d44413d526ec12ed83acd7343c2005782178c7ad implemented this missing
functionality. It set focus to the automatic default button, unless the
QDialog had a focusWidget() set.

That has caused a regression, in cases where
- the QDialog has a QWidget child with a Qt::StrongFocus policy, and
- the QDialog is not yet visible, so focusWidget() returns nullptr.

Amend d44413d526ec12ed83acd7343c2005782178c7ad:

Implement a helper in QWidgetPrivate, that returns true, if a child
with a given focus policy is found.

Do not set focus to a QDialogButtonBox's automatic default button, when
- not located inside a QDialog, or
- a focusWidget() exists, or
- the dialog has QWidget child with Qt::StrongFocus, that is not a
child of the QDialogButtonBox.

Add an autotest function.

Fixes: QTBUG-121514
Change-Id: I3c65ae36b56657f9af4a3a4b42f9b66e8bc5c534
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -13156,6 +13156,24 @@ void QWidgetPrivate::setNetWmWindowTypes(bool skipIfMissing)
 #endif
 }
 
+/*!
+   \internal
+   \return \c true, if a child with \param policy exists and isn't a child of \param excludeChildrenOf.
+   Return false otherwise.
+ */
+bool QWidgetPrivate::hasChildWithFocusPolicy(Qt::FocusPolicy policy, const QWidget *excludeChildrenOf) const
+{
+    Q_Q(const QWidget);
+    const QWidgetList &children = q->findChildren<QWidget *>(Qt::FindChildrenRecursively);
+    for (const auto *child : children) {
+        if (child->focusPolicy() == policy && child->isEnabled()
+            && (!excludeChildrenOf || !excludeChildrenOf->isAncestorOf(child))) {
+            return true;
+        }
+    }
+    return false;
+}
+
 #ifndef QT_NO_DEBUG_STREAM
 
 static inline void formatWidgetAttributes(QDebug debug, const QWidget *widget)

src/widgets/kernel/qwidget_p.h

@@ -737,6 +737,7 @@ public:
 
     bool stealKeyboardGrab(bool grab);
     bool stealMouseGrab(bool grab);
+    bool hasChildWithFocusPolicy(Qt::FocusPolicy policy, const QWidget *excludeChildrenOf = nullptr) const;
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS(QWidgetPrivate::DrawWidgetFlags)

src/widgets/widgets/qdialogbuttonbox.cpp

@@ -1023,8 +1023,10 @@ void QDialogButtonBoxPrivate::ensureFirstAcceptIsDefault()
         // focus proxy/first button stealing the default button status
         // immediately when the button box is focused, which is not what
         // we want. Account for this by explicitly making the firstAcceptButton
-        // focused as well, unless an explicit focus widget has been set.
+        // focused as well, unless an explicit focus widget has been set, or
-        if (dialog && !dialog->focusWidget())
+        // a dialog child has Qt::StrongFocus.
+        if (dialog && !(QWidgetPrivate::get(dialog)->hasChildWithFocusPolicy(Qt::StrongFocus, q)
+                        || dialog->focusWidget()))
             firstAcceptButton->setFocus();
     }
 }

tests/auto/widgets/widgets/qdialogbuttonbox/tst_qdialogbuttonbox.cpp

@@ -6,6 +6,7 @@
 #include <QtWidgets/QStyle>
 #include <QtWidgets/QLayout>
 #include <QtWidgets/QDialog>
+#include <QtWidgets/QLineEdit>
 #include <QtGui/QAction>
 #include <QtGui/QStyleHints>
 #include <qdialogbuttonbox.h>
@@ -82,6 +83,8 @@ private slots:
     void task191642_default();
     void testDeletedStandardButton();
     void automaticDefaultButton();
+    void initialFocus_data();
+    void initialFocus();
 
 private:
     qint64 timeStamp;
@@ -958,5 +961,39 @@ void tst_QDialogButtonBox::automaticDefaultButton()
     }
 }
 
+void tst_QDialogButtonBox::initialFocus_data()
+{
+    QTest::addColumn<Qt::FocusPolicy>("focusPolicy");
+    QTest::addColumn<bool>("lineEditHasFocus");
+
+    QTest::addRow("TabFocus") << Qt::FocusPolicy::TabFocus << false;
+    QTest::addRow("StrongFocus") << Qt::FocusPolicy::StrongFocus << true;
+    QTest::addRow("NoFocus") << Qt::FocusPolicy::NoFocus << false;
+    QTest::addRow("ClickFocus") << Qt::FocusPolicy::ClickFocus << false;
+    QTest::addRow("WheelFocus") << Qt::FocusPolicy::WheelFocus << false;
+}
+
+void tst_QDialogButtonBox::initialFocus()
+{
+    QFETCH(const Qt::FocusPolicy, focusPolicy);
+    QFETCH(const bool, lineEditHasFocus);
+    QDialog dialog;
+    QVBoxLayout *layout = new QVBoxLayout(&dialog);
+    QLineEdit *lineEdit = new QLineEdit(&dialog);
+    lineEdit->setFocusPolicy(focusPolicy);
+    layout->addWidget(lineEdit);
+    QDialogButtonBox *dialogButtonBox = new QDialogButtonBox(&dialog);
+    layout->addWidget(dialogButtonBox);
+    dialogButtonBox->addButton(QDialogButtonBox::Reset);
+    const auto *firstAcceptButton = dialogButtonBox->addButton(QDialogButtonBox::Ok);
+    dialogButtonBox->addButton(QDialogButtonBox::Cancel);
+    dialog.show();
+    dialog.activateWindow();
+    if (lineEditHasFocus)
+        QTRY_VERIFY(lineEdit->hasFocus());
+    else
+        QTRY_VERIFY(firstAcceptButton->hasFocus());
+}
+
 QTEST_MAIN(tst_QDialogButtonBox)
 #include "tst_qdialogbuttonbox.moc"