[mike golding]

I remember I requested something like that quite some time ago for the vertical scrolling. I think an (optional) dotted line where the current page ends and the next begins would suffice for me.

That sounds OK for vertical scrolling and maybe something like that for horizontal page turning might work. But I would rather see a thicker or colored vertical bar.

Yes Mike, I assumed the black bar was just empty space between the sides. The page out of sequence indicator could be anything people want, but I think it should be subtle so it doesn't distract. How about a flashing red marquee around the left side? (kidding of course). I think just a thicker vertical empty space when the left page is ahead of the right (with alternate page turning) would be the simplest and most intuitive, because of the analogous relationship to the separation bar (empty space) in half page turning mode. The bar could be colored, and configurable if people want, but I don't think it's necessary, again maintaining the analogy to half page mode.

Conceivably no space at all between pages in two page mode, as margins and the right and left borders of the score might suffice. Then use the same width bar you have now for the indicator. But since that's probably too much change for users, perhaps just use an even thinner vertical bar when the pages are in sequence.