Until the focus changes to a new record, there's nothing to trigger your script. Scrolling the window doesn't change the focus to a new record.
OnRecordLoad is the trigger that I'd use, but i wouldn't scroll from record to record, I'd use either the book control or my own navigation buttons that use go to record (next, previus, first, last) to move from record to record so that the OnRecordLoad Trigger will be tripped with each mouse click.
I have a phone message taking solution that's part of our contacts manager that is set up exactly that way. A given message taken down may be entered as a "memo"--a large text field takes up most of the screen or as a "msg" where the screen is filled with check boxes and radio buttons that replicates a paper message taking form that they used to use. The OnRecordLoad trigger on both layouts automatically switches layouts to fit the message to the specified layout.