It took me a very long time (longer than I'd like to admit) to learn that the button in the upper right corner of the editor was /the/ way to select the WYSIWYG editor. Even searching for variations of "enable wysiwyg editor" in DDG/Google didn't reveal anything other than there IS an editor available.
I was looking on the edit menu, the tools menu, inside of the options, clicking in the preview pane and trying to edit, etc.
This is very clearly obvious after you've done it the first time, but not obvious at all (I thought it would turn off one pane or the other, not switch modes) until you click it the first time.
Suggestion: Add an option in the Edit or View Menu, Make it an option in the Tools->Options menu, and/or one of the landing spots in the CMD/Ctrl+L rotation sequence - the "wysiwyg preview" is a non-editable landing page currently.
apart from the fact that ctrl-shift-p under osx invokes the command palette and not the editor, there is no toggle_editor command under osx for any key combination. And the toggle_editor_layout command does clearly something else, but does not switch to the WYSIWYG editor.
Read this thread again, what do I overlook ?