If you want to ever vent from the top you will need a full screen.
The half screen slid all the way up does not provide a seal at the CMR that would keep out insects.
I wonder why this window would even offer the ventilation from the top if the screen won't keep the bugs out. I liked that feature as my son likes to have his window open in the cooler weather at night. I thought that feature would allow me to have more security with the windows open at night.