Comment by CGamesPlay

It doesn't in this font? Possibly because of what they talk about where it optimistically gives as much space as possible to succeeding glyphs. You can try it on the page; the code sample boxes are editable. I see no jumping when I type "calming", their example word.

[append] Oh, I do see some jumping when I type "optimized". Honestly, it doesn't seem very jarring to me; the jumping is always right where the edit point is so the added or removed letter causes a much bigger jump.