I plan to make it so that you can open all files when using the "Open…" command (like in Kodex). 

The file registrations for open-in-place and opening files from share sheets in other apps are defined in the Info.plist file of an app. This is a static file included in the app bundle that cannot be edited at runtime, so it cannot be changed when adding custom syntax definitions.

Thank you!

It sounds like you haven't upgraded to Textastic 6 yet. The new version has support for iPhone X.

Is the file possibly stored in iCloud Drive and do you have the same file opened on multiple devices?

Können Sie bitte eine Beispieldatei hochladen, damit ich es hier testen kann?

Oh, please let me know whether this is fixed by the next beta.

Hello, thanks for letting me know. I currently do not run the beta - do you have a screenshot?

Not yet. I believe there was a separate app announced that could do it, but I cannot find it at the moment, sorry.

For Textastic 7, I plan to add the ability to open multiple files in tabs.

Between launches, Textastic will remember which files are opened in tabs. This means, that you can keep your set of files open and you do not even need to select from recent files.

Would that already help you?

I guess by distraction free mode you mean that only the editor and keyboard should be shown and toolbars and other UI should be hidden?

You can already set the column at which the text wraps if you disable the Auto Word Wrap setting. The text won't currently be centered though.