Notify about focus object changes upon widget destruction

If the active QWidget gets destroyed, then QWidgetWindow::focusObject
will return nullptr. If then no other object takes focus, then we'd
never emit change signals, and QGuiApplication's _q_updateFocusObject
(which then informs the input context and emits signals) didn't get
called. This left the input context with a dangling focus object
pointer, which resulted in crashes.

If the QWidget clears its focus, but the corresponding window doesn't
know that it had focus, then fall back to the widget's focus widget
to see if we have a change in focus, so that signals get emitted.

Add a test case that shows that we didn't call _q_updateFocusObject
by counting emissions of the QGuiApplication::focusObjectChanged
signal, which we emit in this function. The signal is emitted more
than once both when showing a widget, and now also when destroying
a widget that has a focus child. The former is a previous issue,
the latter is an improvement to not emitting the signal at all.

Pick-to: 6.3 6.2
Fixes: QTBUG-101423
Fixes: QTBUG-101321
Change-Id: Ib96a397211d442f52ce795a3eebd055a0ef51b0d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 3926436b07..5a17a45c74 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -6707,7 +6707,14 @@ void QWidget::clearFocus()
     }
 
     QTLWExtra *extra = window()->d_func()->maybeTopData();
-    QObject *originalFocusObject = (extra && extra->window) ? extra->window->focusObject() : nullptr;
+    QObject *originalFocusObject = nullptr;
+    if (extra && extra->window) {
+        originalFocusObject = extra->window->focusObject();
+        // the window's focus object might already be nullptr if we are in the destructor, but we still
+        // need to update QGuiApplication and input context if we have a focus widget.
+        if (!originalFocusObject)
+            originalFocusObject = focusWidget();
+    }
 
     QWidget *w = this;
     while (w) {