Didn't see this when I searched...
While in Dual window mode... would be great to be able to highlight in the WYSIWIG window and see the line in the MD window high lighted... that way if you are reading in the WYSIWIG window and you realize you need to make an edit you can get oriented in the MD window to make the update.
A further enhancement would be to allow us to edit directly in the WYSIWIG window while in Dual window mode... I think saw a request for this.
That's not the WYSIWYG panel, that's the rendered markdown panel (that's why you cannot edit in that panel).
But I like the idea of highlighting some text in the rendered view to quickly locate the same spot in the markdown panel.
The WYSIWYG editor is separate, and an alternative editor to the markdown editor.
Nice idea!
When I think about it, I'd directly suggest a small enhancement: Be able to click somewhere in the preview pane and have the cursor in the markdown editor pane jump to the according location, optionally with some small animation (like the tap visualization on some mobile devices) to give a visual indication to the user where the cursor is now located.
Then if I find some missing word in the preview, I could just click there, cursor would jump to the according position in the editor and I could start typing.