It should be trivial to get the resize functionality back. I think in cases like this breaking existing functionality is fine. This is obviously aimed at people with a relatively narrow use-case in mind. In a situation where the purpose of full screen mode is to remove the distractions of the surrounding document this seems fine. I personally would want my text box at a fixed size on some forms with the option to make it the only thing on the page (case in point is my own app https://writeapp.me [yes, an account is needed to see the text box]) so as to keep the design from breaking (it's a very opinionated page when it comes to design).

I think overall this is an improvement to text boxes generally which is why I can forgive the breakage of one bit of existing functionality.