I'm also a heavy Obsidian user, but yes it's a full featured markdown editor.

Heynote does exactly what you like from notepad++ (persistent buffer, no saving) but instead of tabs you can press ctrl+enter and just start a new note (with line numbers starting over at 1) right below the previous one.

On top of that, hit ctrl+L and you can pick your language and get some nice formatting.

All just as lightweight as notepad/++