This is definitely something that can be improved.
The problem is that all lines in the file need to be laid out again to calculate the new line breaks. For a large file this can take some time and happens in a background thread.
I think I could add code that scrolls the editor once layout is finished.
Due to changes in WebKit on iOS 13, scrolling in the SSH terminal does not work anymore (it worked in iOS 12 and earlier). I already spent a few days trying to make it work again, but I could only get either text selection or scrolling to work, but not both at the same time.
I hope I can fix this in a future update. It may require a rewrite of the whole terminal engine.
Do you know that you can add custom themes to Textastic? See https://www.textasticapp.com/v8/manual/customization/custom_syntax_themes_templates.html
You can find many themes including Tomorrow Night at http://tmtheme-editor.herokuapp.com/#!/editor/theme/Tomorrow-Night (you need a desktop browser for this though).
I currently do not plan to add more localizations, sorry.
You can swipe left and right on the editor to move the cursor.
You can also tap with two fingers on the editor to bring up the cursor navigation wheel. See https://www.textasticapp.com/v8/manual/viewing_editing_files/cursor_navigation_wheel.html
support for adding external folders needs to be implemented by the file provider. It currently works with iCloud Drive, "On my iPad/iPhone", the Git client Working Copy, and the SFTP client Secure ShellFish.
Other file provider apps (like Google Drive or Dropbox) need to be updated to support this.
You can however use the "Open…" command to open single files from these file providers.
As an alternative you can use Textastic's built-in Dropbox and Google Drive clients to download whole folder hierarchies:
You should however be able to use the "Open…" command in Textastic to open individual files from Synology Drive.
For this to work on iOS 13, the file provider, in this case the Synology app, needs to add support for opening folders.
I know that this currently works with iCloud Drive, "On my iPad/iPhone", Working Copy (Git client), and Secure ShellFish (SFTP client).
File providers that do not yet support this are greyed out when trying to use the "Add External Folder…" command in Textastic.
iCloud pretty much does its magic in the background. For more control you should consider using other methods like the built-in SFTP client. Using it, you can manually download and upload files. See https://www.textasticapp.com/v8/manual/remote_servers/sftp_ftps_ftp.html
Since you seem to be familiar with Git, also have a look at the Git client app Working Copy - Textastic can open Git repositories from Working Copy as external folders: https://www.textasticapp.com/v8/manual/integration_other_apps/git_client_working_copy.html
Customer support service by UserEcho