While your mockup is interesting, I’m not convinced that menus in menus are a good thing. In fact, I have rarely seen one ever since Ribbon UIs or Touch screens were a thing. I’m also wondering how well it would work on mobile…
Could you also explain what those glyphs at the bottom are supposed to mean? The keyboard is somewhat logical for keyboard shortcuts and the question mark could stand for FAQ or Help, but the Phone glyph is puzzling.
That represents the Mobile View / Desktop View switcher. If it’s unclear, then it should probably be a text link instead.
My reasoning was, since the list of categories is very long and not frequently accessed, it’d be nice to make it expand as needed. This gets rid of the bulk of the menu when it’s not needed.
This was also mentioned in codinghorror’s suggestion:
There’s probably a better way to expand the categories instead of a dropdown, but this exact one is already being used and works fine on mobile.