When you open the site https://lmarena.ai/ goes to the "Direct Chat" tab makes the model selection blocks, the chat window and the query input block fixed above all other elements.
Everything this script does is in the description.
I can only add why it was decided to make this script. It's simple - I wanted to be able to focus on communication, without unnecessary elements and without scrolling to the buttons for selecting a model or entering a request.
And yes, the chat window width is limited to 900px
