The mou/macdown/classeur approach is to have a preview that can be toggled or visible in split screen mode. Conversely, notes.app on Mac works really well and you can write and read notes on the same "space" but it doesn't support markdown.
Maybe I am an edgecase, but I would like to write notes in markdown and have them render line by line[0] so that I have one document and not an editor and a display. Almost wrote one myself but the opportunity cost of spending weeks making it (if I could even deliver) would likely not offset the bit of frustration I get using another editor.
If this exists, I want it bad.
[0] or like 3 lines at a time or whatever.