More slide thumbnails in notes

Originally reported by: michaelaoash (Bitbucket: michaelaoash, GitHub: michaelaoash)

This request would apply to material on page 176 of the User Guide v. 3.06.

I love giving talks with the notes on the second screen. It's one of the best of latex-beamer's many excellent features.

It would be neat to be able to see thumbnails of the current slide, next slide, and perhaps the previous slide at the top of the notes on the second screen. In the current version, the notes screen displays a thumbnail of the current slide, but it would be really nice if the default were to display thumbnails of the current slide and the next slide.

An alternative would be to modify \insertslideintonotes so that it is possible to specify an arbitrary slide number.

This would be very helpful because it could cue remarks such as, "On the next slide I will show you..."

• Bitbucket: https://bitbucket.org/rivanvx/beamer/issue/5

Start with the notes-screen behavior described on page 176 of the User Guide v. 3.06 and inspect the \insertslideintonotes entry point named in the issue. Determine how the current-slide thumbnail is produced and define whether the finished change should show the current and next slides by default or allow arbitrary slide numbers, then verify the notes display accordingly.