@mgmeyers great work! You list https://www.speare.com as a possible inspiration for Kanban. Revolutionary in the process of writing by going granular and keeping the big picture at the same time. Definitely a valuable idea to include this as a Kanban-feature!!!
https://gingkowriter.com/ is another example ( less kanban but great! https://youtu.be/J4prcx0jZ9M)
Already mentioned here under Document Spanning: