Hi Alistair,
You're spot on with the reasoning! I must admit I hadn't really thought about that issue.
I'm concurrently reworking the app to be a nicer Material themed, and hopefully more intuitive UI. This will have many redesigned elements and the one I was looking at was how to display the 'extra information' like the 'Next in set" text. I think I will be adding this as an overlay in this version, so this won't affect the size of the highlighter/song display. I think I'm not going to try to fix this in the current version though if you're ok with that? You can temporarily work round this by creating the highlighter notes once the song is displayed in a set (so the bottom info space is already there). However, that will mess up the alignment if the song isn't in a set. Alternatively switch off the display next in set (although I must admit I find that info very useful!).
Best wishes, good to hear from you and thanks for alerting me to this!
Gareth