I've been looking for a replacement to Mou for a while, and I'm excited to see Haroopad offering not only Markdown support but also a bunch of other features (especially MathJax and intelligent autocomplete). I'll definitely try it out.
After further examination, a number of the features claimed on the website seem to be missing (at least on Windows).
I don't get PDF export, math mode is not enabled by default (and enabling it breaks the super/subscript text-mode notation; they're apparently mutually exclusive), the social media embedding doesn't seem to work either (I tried embedding a youtube video and it just puts up a throbber). This shows a lot of promise, but is very broken right now.
Also, the preferences dialog is buried in the "File" menu ... which I don't think is a standard location on any platform.
It's turned off by default in the preferences dialog (which is in the File menu for some reason...) Turning it on breaks some of the other notations (like the text-mode super/sub-scripts).
I'm under the impression the authors are Korean; it's the only language besides English unattributed in the translations list, and the English localization has a few rough edges suggesting non-native speakers.