All the Text functions assume the cursor is always on a text line. I was violating that invariant.

• When scrolling up, I start the cursor at the top-most line below the screen top.

• When scrolling down, I start the cursor at the top-most line below the screen bottom.

  I think it would feel slightly more natural for it to be the bottom-most line above the screen bottom.

  However, the Text functions maintain an invariant that the bottom-most line in a buffer will be text. There's no such invariant for the top-most line.