I already use notepad++ this way. Mostly, it is my scratch pad for stuff until I figure out what to do with it (save in Evernote, create an issue, send email or slack message, etc).

The killer feature that enables this is if you create a new document, it stays there (as "new2" or whatever) until you explicitly close that document. Closing notepad++, rebooting, etc doesn't matter - it's still there next time you open notepad++.

I've tried with vscode, but its behavior of closing everything if you open a folder just makes it not work for me, even though I often use it for coding.

You wouldn‘t have the browser environment with all md libs there, should be somewhat harder