Your comments

Are you on iOS 13? I'm asking because there is a bug in iOS 13 so that touch scrolling does not work.

Ah, ok, you need to add Working Copy as a location in the Files app for this to work. Otherwise the file provider isn't active. I haven't tried it without that to be honest.

I just tried it on an iPad Pro running iOS 13 beta 7 and it works for me. Please note that you need to drag a repository from Working Copy (not from the Files app) into the sidebar of Textastic (where the "Local Files" and "iCloud" locations are shown).

The latest TestFlight beta of Textastic also has a new "Add External Folder…" option on iOS 13.

You can add iCloud Drive -> DEV as an external folder. Have a look at the manual at https://www.textasticapp.com/v8/manual/integration_other_apps/external_files_folders.html for details.

Yes, currently files are deleted immediately.

I'll try to find out whether it's possible for an app to move a file to recently deleted instead of deleting it immediately.