When we rebuilt the Forestry.io markdown editor we decided to use Prosemirror. In the two years since we’ve been incredibly happy with how it’s worked out!
A couple months ago we released a new open source project called TinaCMS. When it came time too build the markdown editor, we knew we’d go with Prosemirror again.
You can find the source code for the TinaCMS markdown editor here
(P.S. If anyone is interested in helping make the TinaCMS wysiywg even better then drop me a line 
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