I was able to modify the width of a stubborn react-admin component dynamically, using another DOM element as a reference point, and it was actually pretty clean! Here’s a stripped-down version:

  MyComponent = () => {
    [width, setWidth] = useState()

     useEffect(() => {
        const main = document.querySelector('main');
        if (!main) {
          return;
        }
        setWidth(main.getBoundingClientRect().width - 50);
     });

    render <StubbornComponent style={{width}}/>
  }

I would’ve preferred a pure-CSS solution, but this seems to be working and the code is temporary (well, moreso than usual, hopefully)