QWebEngineView with Qt::NoContextMenu policy fails to defer contextmenu event to parent

If a widget has Qt::NoContextMenu set, then

the widget does not feature a context menu, context menu handling is deferred to the widget's parent.

From https://doc.qt.io/qt-5/qt.html#ContextMenuPolicy-enum

Just, QWebEngineView does not follow that specification, instead it just swallows the event, so the context menu on the parent widget is not instead invoked.

Reason seems to be that bool QWebEngineView::event(QEvent *ev) has an implementation which always accepts events of type QEvent::ContextMenu, so no deferring to the parent ever happens, including the case of Qt::NoContextMenu.
See http://code.qt.io/cgit/qt/qtwebengine.git/tree/src/webenginewidgets/api/qwebengineview.cpp#n317

A fix would be to make that method follow instead the behaviour of bool QWidget::event(QEvent *event) and only accept the event in case of matching policies, and ignore in case of others, incl. Qt::NoContextMenu.

See http://code.qt.io/cgit/qt/qtbase.git/tree/src/widgets/kernel/qwidget.cpp#n8981