I'm also a believer in in-place formatting rather than 2-pane. Ever since i saw it in [Qute][] (no longer developed), and emacs auctex preview-mode (TiddlyWiki also deserves some credit).
Editorially did this well and was well-recieved but for some reason people are not really filling the void.
I'm especially surprised it's not more popular on mobile where 2 panes waste real estate and whole-screen toggling is jarring. The only mobile editor with in-place formatting I know is Writer & [Writer Plus][].
[Qute]: http://www.inkcode.net/qute
[Writer Plus]: https://play.google.com/store/apps/details?id=co.easy4u.writ...
Also,
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I'm working on https://mathdown.net [https://github.com/cben/mathdown] which does in-place formatting. It's comparable to Stackedit or Classeur in that it's simply "syntax highlighting", e.g. headings are big and emphasis is italic.
- The formatting chars (#, * etc) are still visible, just a bit de-emphasized. I know many people prefer them to disappear except near the cursor. That's trickier to implement with good UX, and not planned soon (PRs welcome of course).
- There is no inline image preview (yet). Nor tables, diagrams, embedded videos... Even links are not (yet) clickable.
+ There is nice in-place LaTeX math rendering (slighly buggy but very usable). This is my biggest unique feature [https://github.com/cben/CodeMirror-MathJax]. Comparable to Overleaf.com's "rich text" mode.
+ I'm doing "soft indent" [https://www.mathdown.net/?doc=demo-indentwrap]. This is a really low-hanging way to make one-long-line-per-paragraph usable, which IMHO is essential for in-place styling.
+ Leading indentation (and list bullets/numbers/quotation) is monospaced to help align stuff.
If anybody's building their own, I'm using and strongly recommend CodeMirror. It makes mixed fonts - things like bigger headings - really trivial and supports arbitrary embedded "widgets". @benweet pulled off polished in-place formatting in StackEdit (and on top of contenteditable but that's way harder...