I'm gonna go with "it's a bad idea".
Generally when a user opens the menu they're trying to go to a different page.
But there are other issues like bookmarking or sharing the page with the menu open. People won't expect the result.
It's really not difficult to create an on-page menu so this seems more trouble than it's worth.
I think it is a beautiful, simple solution that just works (and also did work perfectly in the past when we didn't have JS)