Preserve tabs in source edit.

Drupal 8. Editing content as source instead of content. I make a change then save the change. Next time I open the content as source, the content is changed. Is there any way to stop the editor overriding changes to source?

In this case I have whitespace, spaces and tabs, inside and outside pre elements. The whitespace is retained inside the pre element and changed outside the pre element. This makes the content source different from the content source file which creates a problem managing the content source.

The alternative appears to be to add another option to the edit list. Add “Source edit” after the “Full HTML” option so that all editing is always in source mode. It would have to be available only for some content types and one role. How do you do that in Drupal 8?

Drupal version: