This is a Samsung specific feature, it is not related to Textra SMS. 

This Samsung feature is only intended for apps that support Samsung multi-window, which Textra SMS doesn't.

Either disable this feature, see Android Settings > Advanced features > Smart pop-up view > Textra SMS = OFF.

Or configure it to mostly work (it's flaky at best), see Textra > Settings > Customize Notifications > Define Notification Actions > Tapping Opens = please select Quick Reply no Keyboard.