In addition to standard tabs, sliding panes aka @andymatuschak mode would be great.
I’d recommend looking at the way the Zed app does this. Very slick. Nice, clean, and intuitive. Drag and the window shades the regions to drop the file (top, bottom, left, right). Very slick!
That UI makes sense if you ‘leaf’ through files, like your notes. Lettera is positioning itself more general than that.
I concede that it’s a niche way of working. The purpose, at least for me, is to offer a helpful way to look at more than one note at a time while working.
I read what you said here and understand.
An alternative to sliding panes that I’ve found really helpful is in Roam Research. There you can shift-click on a block or link and it opens in the right-side pane (and you can open more than one).