Hi AJ Valenty, thanks for your reply.
My original post was referring to page scrolling, as opposed to line-by-line scrolling. I tried line-by-line scrolling as well, and the behavior is even worse: nothing moves, and the blue selection square disappears after the first key press.
I noticed your video was recorded in Chrome, so I tested with Chrome on macOS Tahoma, and everything works correctly there — both page scrolling and line-by-line scrolling function as expected.
This suggests that either Safari (my default browser) has a bug, or the Graphite UI doesn’t fully support Safari. Is this information sufficient for you to investigate and determine which side the issue is on?