Page up and page down fail on first attempts in DemoScroll

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.

https://github.com/user-attachments/assets/e4e027af-0741-4f4d-97fa-d99b3a6c358b

Reproduce the sequence in DemoScroll: select 665/57 with the arrow keys, then press Page Up and Page Down while observing the flicker and reselection. Start by locating DemoScroll's keyboard and selection handling; done means the first Page Up and Page Down actions work correctly without the selection disappearing.

Written by the indexing model from the issue text.