I just got a notification that the WebKit team are finally looking into this rendering bug that has blocked my ebook reader project since Feb 2025 🥹
I filed a Bugzilla report on Sep 2025. There are at least three parts to fix, but it sounds like they may have fixed one of them!
My app continuing to hit WebKit rendering bugs:
Place a <div style="position: fixed"> into your <body>. That <div> will scroll just fine *until* you scroll the <body> along by one viewport, whereupon it cannot be scrolled.
… but *only* for `writing-mode: vertical-rl`! 😭