I’m not sure if this has been covered before but in my search for the best Markdown Editor that can be used with Joplin, I found this plugin for Notepad ++ that will give you a Mardown WYSIWIG window to the right of your input screen. Similar to what Joplin does natively, but I like the look of the MarkdownViewer ++ Plugin that can be used with Notepad ++.
The Plugin can be found here:
Just copy the DLL file to your Notepad ++ Plugin Directory and then install the Plugin via the Plugin Admins option with Notepad ++. Find MarkdownViewer ++ and select it and then click on the Install button. Notepad ++ will install the Plugin, Close down and relaunch itself.
To modify the options of MarkdownViewer ++, In Notepad ++ select Plugins from the Menu | MarkdownViewer ++ | Synchronize Scrolling (Editor/Viewer).
There are other options to select under Plugins | MarddownViewer ++ | Options
There is a readme.md file within the zip file that explains the nuances of MarkdownViewer ++.