I'm not seeing this, but it may depend on which font you are using.
Most likely, the height of the StageText used by StageTextTextEditor is being set too small. The problem is that StageText doesn't provide any kind of measurement API, so it falls back to using a TextField. Since they render text slightly differently, the height reported by TextField may not be accurate for StageText.
There's not really anything that can be done automatically by Feathers without Adobe providing a more robust StageText API. You're probably going to need to set the height of StageTextTextEditor manually. Maybe you can check what it is, and then hard code it in with an extra pixel or two.
Alternatively, you might be able to switch to TextFieldTextEditor. That one should accurately measure, so I wouldn't expect to see a scroll bar.