Forget half swipes. Use notification previews instead.
On iPhone
- Settings → Notifications → Snapchat.
- Turn on Lock Screen alerts and Show Previews.
- Now read most messages from the lock screen without opening the app.
On Android
- Settings → Notifications → Snapchat.
- Enable lock screen notifications and message preview.
You read content from the shade or lock screen.
Snapchat will not mark the chat as opened until you tap into it.