Looks and feels great.

My instinct was to scroll my trackpad to go prev/next page, but scrolling seems to iterate pages as fast as the app can possibly load them.

Speaking of language learning, I was hoping to use macOS built-in dictionary when highlighting a word but I see that you've offered your own popover. I don't know if there's a solution that offers both built-in OS stuff and custom functions when you highlight content.

Just trying to think of feedback. The UX is really polished and fast. And I generally like the minimal VS Code style.