In #13 I made this fix for left margins:

just shifting the content over every time it's displayed. This is less performant, but I don't know if it would make much of a difference.

It did end up making a difference, on long documents like gemini://tilde.black/users/fox/irc/log.txt. It takes a long time to load, which is one thing, but also even once it's cached, it takes 20 seconds to view it after navigating away from it and coming back. This is because the left margin spaces are being added to the entire document again. One solution would be to have a Page.LeftMargin integer attribute. And that's only recalculated when a reload happens, which would set it back to -1 or something.