Your comments

The ctrl-k functionality is really not at all emacs-like at the moment, and ctrl-y to yank back what was deleted is missing.

For example, given 3 non-empty lines, with the cursor positioned at the beginning of the first line, press ctrl-k, it appears to work correctly (deletes to the end of line, but not the newline itself). But then press ctrl-k again, and it deletes both the newline and the second line. Each ctrl-k should delete the text to the end of line, then the newline, then the next line text, then the next newline, and so on.

Also, each subsequent ctrl-k should append to a kill buffer that can be yanked back with ctrl-y. Emacs has much more involved behavior than this (and I am not suggesting Textastic should do all that), but even TextEdit, XCode, and Pages (in fact, most apps) all do this correctly. Please fix Textastic to work like this on the Mac, also.

Nice editor, otherwise.