Page up and page down fail on first attempts in DemoScroll

[cursork]

I use the arrow keys to select 665/57, then I press page up and my selection flickers on 661/57 and disappears, I then press page down and it flickers on 670/57, before reselecting 665/57 and thereafter it works exactly as expected.

I believe this worked previously, but there's always a chance I was mistaken.

output.mp4