Focus is a bit wonky as of Textual v0.20.x

[davep]

I'm finding I can't easily get focus into the activities, using just the keyboard. I suspect that this is down to how VerticalScroll handles being focused these days. Given that this is one of my older Textual applications and did do some fun things with focus management back in the early days, I should revisit the flow of focus and the code I use to manage it.