Same in Safari. It has something to do with the

  :root {
    […]
    overflow: hidden scroll;
    container-type: size;
    […]
  }

in the main CSS file: https://kellett.im/theme/main.css

Splendid. Someone found a way to break Browser Scrolling. (Firefox 115.16 for Win7)

Well done.

Cannot scroll on Safari on macOS, either. What also doesn't work is making the font smaller / larger.