To the best of my knowledge this (Marktext) is the only free/open-source Markdown editor that has WYSIWYG (like Typora: https://typora.io/). Not a separate preview window, but the actual editor window renders each Markdown "block" (paragraph, list item, whatever) except the one you're editing.
I've been working on one (https://github.com/blackhole89/notekit) for a while now (which, unlike the aforementioned, also is not built on Chrome/Electron). Unfortunately it seems to be pretty hard to get the word out, or at least I haven't found any better strategy than to pounce, as I am doing now, whenever I see a HN thread about markdown editors (which invariably wind up having some comment thread lamenting the lack of WYSIWYG and/or non-Electron editors).