Slide In plugin has persistent CSS error

I have installed and want to use your slide-in marketing/email signup plugin. As per attached screen shot, the plugin doesn't render correctly. I've tried all manner of things, positioning, changing the colour, etc. but the tab has this artifact that sticks out. This appears on Windows, Mac, various browsers. Am using simple gravity form short code. Cannot provide an URL.

I think it's a padding issue. Have spent far too much time to find the error in the styles, but could not fix it. Can't spend any more time on this, and I hope you can help. Please advise.