Using multi byte characters allows you to scroll too far, not stopping you from seeing all the text.
So i'm fine with it being a known limitation (Freedom?).

Added longest line length to the drawcache so we don't need to loop over all lines every draw call.

Commandeering to try and update when I'm the author.

Hmm you're right.
However I think looping over visible lines only will cause weird behavior e.g. Jumping left horizontally when scrolling away from the longest line.
I added longest_line_length to the drawcache instead.

Hey, I have a diff ready here.
What happens now?

Hey, I started on this one last weekend and I have a working version of the drawing functions - The scrollbar is correctly drawn according to line length and window size and so on.
I'll get the code to a presentable state and add mouse functionality over the next few weekends.
Just a question though:
What should the highlight be when the selection spans more than one line?
For example if "according to line length" to "I'll get the code" was selected, should there be two red line at the bottom? Should there be a red line at all?