As you can see, in the bottom right corner of the picture I managed to highlight a belt even though being in the menu. How to reproduce:
- set game to borderless
- build some machines (so you have anything to hover)
- escape to menu
- tab out of the game so it loses focus but is still visible
- [edge case bug]: (without giving the game focus) you can now hover machines through the menu blur, they become highlighted.
- click back into the menu so it gets focus again, suddenly machines aren't highlighted anymore when you hover them
I assume the hover/highlight logic somehow skips the menu screen if the game loses focus.