Messes up page

This is a really useful project and I was excited to try it out.
As soon as the script is activated the textarea's height is changed and it creates a sort of mirroring effect with the highlighted and uneditable text above and the normal text in the textarea below.

I've taken some looks at the code and it seems as though the highlight elements adapts the textarea's CSS and I think this is what's caused the issue. I was able to manually change the CSS and I got close to making it work and look good, but once the page is resized it gets messed up again.

Any ideas on what the cause might be?