Are the internals of other Mac-native text editors like TextMate similar to this as well?
Depends on what you call “similar”. They’ll have storage, layout, a part of the layout that’s visible, etc, but they may not separate them that clearly, and won’t have that rich an API. TextView is way more flexible than what the typical _text_ editor needs. Text editors typically don’t need multiple fonts, multiple font sizes, varying line spacing, paragraph indents, proportional fonts, right-aligning or decimal tab…