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>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 353a80d1c5..04290a4ce1 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/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;
+    }
 }