Clarify how key / mouse QEvents are propagated to a popup RWHVQD object

Change-Id: I74f6a607d96733fa379526be40c6a13c31203d4e
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/webenginewidgets/render_widget_host_view_qt_delegate_widget.h

@@ -48,6 +48,12 @@
 
 namespace QtWebEngineCore {
 
+// Useful information keyboard and mouse QEvent propagation.
+// A RenderWidgetHostViewQtDelegateWidget instance initialized as a popup will receive
+// no keyboard focus (so all keyboard QEvents will be sent to the parent RWHVQD instance),
+// but will still receive mouse input (all mouse QEvent moves and clicks will be given to the popup
+// RWHVQD instance, and the mouse interaction area covers the surface of the whole parent
+// QWebEngineView, and not only the smaller surface that an HTML select popup would occupy).
 class RenderWidgetHostViewQtDelegateWidget : public QQuickWidget, public RenderWidgetHostViewQtDelegate {
     Q_OBJECT
 public: