There’s been a slight change in the UI design for Navigation in 1.8. In previous versions, the Navigation icon (the icon with 3 horizontal stripes, a.k.a. the “Hamburger” icon) was permanently displayed in the top left of the screen. Clicking it would reveal the main Navigation menu. By default, this is still the case in 1.8:
However, in 1.8, it is now possible to have the Navigation menu permanently displayed instead (Settings > General > “Always show the sidebar” set to yes).
When this is done, the Navigation sidebar is permanently shown, and the Navigation icon is not needed.
You have the setting set to “Yes”. If you want to revert the Navigation menu to act as an overlay, rather than permanently take up space on the Roon screen, set this to “No”.