Difference between QWidget::grabKeyboard() and the grab done by QMenu

Question

I noticed that while a QMenu is open, my key bindings stop working ( xbindkeys ), also screensavers can't start and report a "Can't grab keyboard" error.

This doesn't happen with other QWidgets that use QWidget::grabKeyboard().

Does QMenu grab the keyboard, and if so, is it a QWidget::grabKeyboard() or a lowerlevel X11 thing? If it's a lowerlevel function, why can't it just use a normal Qt keyboard grab.