Mike Stein made the good point that when the buffer is empty it wasn't
clear if the blue area was the positive scrollbar or the negative space
within the scrollbar area.
You can either drag the scrollbar or click anywhere in the scrollbar
area.
It has this weird funkiness that the scrollbar shrinks as you get near
the bottom. But it feels.. weirdly satisfying?
Not quite ideal: the scrollbar computation only considers
screen_bottom1.line and not screen_bottom1.pos, and so it always assumes
the final line is at the bottom of the screen.
I'm making a deeper change here that I might come to regret. I want to
avoid creating new book-keeping for editor mutations, so I'm putting the
work of computing scrollbar data into clear_screen_line_cache. But that
implies the editor should never clear before updating data, and I caught
one place that wasn't true before. A better name helps avoid this in
future. Let's see how much toil this causes in resolving conflicts.