fix(qt6): Fix infinite loop crash in QQmlProxyMetaObject

Qt6 port is now complete, as its tests all pass with this fix.

Don't emit signals in an extension after its target widget dtors have
already started.

QQmlProxyMetaObject doesn't like its parents meta-objects offsets
changing.

Author: iamsergio

src/CMakeLists.txt

@@ -60,7 +60,8 @@ add_library(
 target_link_libraries(
   ${TARGET}
   PRIVATE Qt${QTMAJOR}::Core Qt${QTMAJOR}::CorePrivate Qt${QTMAJOR}::Qml
-          Qt${QTMAJOR}::Widgets Qt${QTMAJOR}::QuickWidgets)
+          Qt${QTMAJOR}::Widgets Qt${QTMAJOR}::WidgetsPrivate
+          Qt${QTMAJOR}::QuickWidgets)
 
 if(Qt${QTMAJOR}WebEngineWidgets_FOUND)
   target_link_libraries(${TARGET} PRIVATE Qt${QTMAJOR}::WebEngineWidgets)

src/declarativewidgetextension.cpp

@@ -39,6 +39,7 @@
 #include <QLayout>
 #include <QQmlInfo>
 #include <QWidget>
+#include <QtWidgets/private/qwidget_p.h>
 
 class WidgetContainerDelegate : public DefaultObjectContainer
 {
@@ -207,6 +208,15 @@ bool DeclarativeWidgetExtension::eventFilter(QObject *watched, QEvent *event)
   Q_ASSERT(watched == parent());
   Q_UNUSED(watched); // release builds
 
+  if (auto w = extendedWidget()) {
+      // This fixes infinite loop QQmlProxyMetaObject::metaCall()
+      // Don't emit signals in an extension after its target widget dtors have already started.
+      // QQmlProxyMetaObject doesn't like its parents meta-objects offsets changing
+      QWidgetPrivate *priv = QWidgetPrivate::get(w);
+      if (priv && priv->data.in_destructor)
+        return false;
+  }
+
   switch (event->type())
   {
   case QEvent::Move: