-- static properties of the code
All strings are UTF-8. Bytes within them are not characters.
I try to label byte offsets as _offset, and character positions as _pos.
For example, string.sub should never use a _pos to substring, only an _offset.
Wish I had some static typing here. We're not going to try to write tests to catch issues like this.

-- manual tests
drawing
  draw a line, circle, rectangle, square, polygon
  select a point and move it
  select a point and name it

persistence:
  draw a line, circle, rectangle, square, polygon, quit, restart. All the shapes you drew should still be visible.
  select a point and name it, quit, restart. Name is still visible.

undo:
  naming points
  deleting points
  moving points

resize:
  create a file containing a long line of characters without spaces. try
  resizing the window vertically and horizontally, as far as possible.