Fix crash when the focus widget gets a focus proxy after the fact

QApplicationPrivate::focus_widget became a dangling pointer
in the following scenario:

A widget first gets focus and later on gets a focus proxy.
QApplicationPrivate::focus_widget was still pointing to the initial widget.
Upon destruction, QWidget::hasFocus() [which follows to the focus proxy
and then compares with focus_widget] was therefore false for both
widgets. So QWidget::clearFocus() didn't call
QApplicationPrivate::setFocusWidget(0) for either of them. As a
result, focus_widget remained set, and became dangling.

In real life, this happened with a QWebEngineView, which the application
gave focus to upon creation. At that time it doesn't have a focus proxy
yet. That happens later, in QWebEngineViewPrivate::widgetChanged.

https://bugs.kde.org/show_bug.cgi?id=381793

Change-Id: Ifee610bb76a2d4d2797b98ece9bffe5fffe3c6a6
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -6436,8 +6436,18 @@ void QWidget::setFocusProxy(QWidget * w)
         }
     }
 
+    QWidget *oldDeepestFocusProxy = d_func()->deepestFocusProxy();
+    if (!oldDeepestFocusProxy)
+        oldDeepestFocusProxy = this;
+    const bool changingAppFocusWidget = (QApplicationPrivate::focus_widget == oldDeepestFocusProxy);
+
     d->createExtra();
     d->extra->focus_proxy = w;
+
+    if (changingAppFocusWidget) {
+        QWidget *newDeepestFocusProxy = d_func()->deepestFocusProxy();
+        QApplicationPrivate::focus_widget = newDeepestFocusProxy ? newDeepestFocusProxy : this;
+    }
 }
 
 

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -181,6 +181,8 @@ private slots:
     void tabOrderWithCompoundWidgets();
     void tabOrderNoChange();
     void tabOrderNoChange2();
+    void appFocusWidgetWithFocusProxyLater();
+    void appFocusWidgetWhenLosingFocusProxy();
 #if defined(Q_OS_WIN) && !defined(Q_OS_WINRT)
     void activation();
 #endif
@@ -2031,6 +2033,51 @@ void tst_QWidget::tabOrderNoChange2()
     QCOMPARE(focusChainBackward, getFocusChain(&w, false));
 }
 
+void tst_QWidget::appFocusWidgetWithFocusProxyLater()
+{
+    // Given a lineedit without a focus proxy
+    QWidget window;
+    window.setWindowTitle(QTest::currentTestFunction());
+    QLineEdit *lineEditFocusProxy = new QLineEdit(&window);
+    QLineEdit *lineEdit = new QLineEdit(&window);
+    lineEdit->setFocus();
+    window.show();
+    QApplication::setActiveWindow(&window);
+    QVERIFY(QTest::qWaitForWindowActive(&window));
+    QCOMPARE(QApplication::focusWidget(), lineEdit);
+
+    // When setting a focus proxy for the focus widget (like QWebEngineView does)
+    lineEdit->setFocusProxy(lineEditFocusProxy);
+
+    // Then the focus widget should be updated
+    QCOMPARE(QApplication::focusWidget(), lineEditFocusProxy);
+
+    // So that deleting the lineEdit and later the window, doesn't crash
+    delete lineEdit;
+    QCOMPARE(QApplication::focusWidget(), nullptr);
+}
+
+void tst_QWidget::appFocusWidgetWhenLosingFocusProxy()
+{
+    // Given a lineedit with a focus proxy
+    QWidget window;
+    window.setWindowTitle(QTest::currentTestFunction());
+    QLineEdit *lineEditFocusProxy = new QLineEdit(&window);
+    QLineEdit *lineEdit = new QLineEdit(&window);
+    lineEdit->setFocusProxy(lineEditFocusProxy);
+    lineEdit->setFocus();
+    window.show();
+    QApplication::setActiveWindow(&window);
+    QVERIFY(QTest::qWaitForWindowActive(&window));
+    QCOMPARE(QApplication::focusWidget(), lineEditFocusProxy);
+
+    // When unsetting the focus proxy
+    lineEdit->setFocusProxy(nullptr);
+
+    // Then the application focus widget should be back to the lineedit
+    QCOMPARE(QApplication::focusWidget(), lineEdit);
+}
+
 #if defined(Q_OS_WIN) && !defined(Q_OS_WINRT)
 void tst_QWidget::activation()
 {