Enable/Disable the “full-size” editor in frontend?

I have the problem that my theme´s header is over your post-editor in frontend when i click on the black full-size button. I use the theme "OnePress". So if the forum content is to short and you can´t scroll, the user can´t klick on the Submit topic or Discard or Cancel-Button. It would be great if i could enable or disable the "black-fullsize-button". I think another great solution be if you can move the editor in the height. I mean if i disable the full-size-button that there will be a button to size the editor in frontend in his height. 

Such a feature would be a great solution for me.