Responsive sign-up form built with React

I'm currently working my way through the 'React Basics' course on Scrimba.com and I wanted to put into practice what I've been learning about React and forms.

I welcome any general feedback for best practices, however I did notice a strange 'glitch' when I was testing this solution. Whenever I input incorrectly formatted data into the form my error messages display correctly (which is great! that piece took some time), but in Firefox and Chrome the form changes it's width when this happens. I tried investigating why this happened and how to fix it, but couldn't figure it out. Any suggestions would be much appreciated. Thank you.