Looks like there have been longstanding issues with using curses to write to the right-/bottom-most character position of a curses window; see open Issue8243 and various discussions elsewhere.  Some of the discussions suggest the issues dates back to support of physical terminals where writing to the last character position caused an automatic scroll up of the lines on a screen and suggest it is best to avoid trying to write in that last position.  I'm not a curses expert so whether that advice is still relevant today and whether Textbox behavior should be changed are open questions.  The behavior could just be added to the proposed documentation changes in Issue8243.  Perhaps @twouters has a suggestion.