Glad to see your cabinet has all the "insides" - often the top support is missing.
This cabinet is designed so that the support leg on the door of the cabinet will drop down and extend when open, and the top flap rests on it for support. The top is not made to be left open without that extra support, like some cabinets are.
My only gripe with this cabinet - it uses a different key than other Singer cabinets - it's a longer barrel key, like a lot of china cabinet use - and it's on the long side.