VSCode also supports WYSIWYG using either webviews or their custom editor API. that's actually something we're looking to implement soon!
Do you mean editing in one window and seeing the result in another? With TeXmacs, you edit in one WYSIWYG window.
that is how Dendron currently works. But VSCode supports WYSIWYG functioanlity inline. You can see an example of it here: https://github.com/tejasvi/markless
we're planning on implementing something similar soon!