The default is to display items in the order in which they are encountered in the HTML file (when using <<Prev||Next>>). Is there a method to force the display order to something of my choice?

There is no such mechanism, other than ordering the links in the html into the desired gallery order.

I've considered adding an "index" option to allow re-ordering but I've never found a use-case where link order wasn't a suitable mechanism. I don't want to bloat Floatbox with things that aren't useful. Maybe you can show me a case where such an option would be necessary? (Note that there is already a "randomOrder" option available.)

I doubt that I can make a case to justify adding that feature. On most of my "monthly article archive" pages I like to use the format in the link below and would prefer that the newsletter month increment by one instead of three (go from January to February instead of from January to April). As a compromise I will need to rearrange the months in rows instead of columns.

http://www.freelandville.us/bethelchurch/chimes/

Yes, you didn't quite sell me on the 'index' option with this (almost though). Because in my opinion the months should be arranged in rows anyway. It's the way we read English things naturally - left to right, not top to bottom.

I'm sorry if I didn't understand the topic right, I'm from Holland.

I think that the statement was that there's no need to an index option?

I do have a plausible reason: I work with php-includes (and more people will be, I assume).
This means you have to upload div's/images as: 1-3-2 to get 1-2-3 on the screen.
Working with an Index would give the possibility correct the display in a group from 1-3-2 to 1-2-3.

I know this topic is of 2011 but I didn't find a solution yet, or is there already one built in that I don't spot?

There's been a couple of requests for an "order" or "index" option, and if folks need it, I'll do my best to provide it. I'm currently banging away on version 6.0.0 and will look at adding this new option to that upcoming release.

I'm not convinced it's any easier to write individual 'index' assignments to each element than it is to write those elements to the page in the desired order. But I don't need to be convinced. If you say you want it, and it's not silly or excessively complex, you'll get it.

Thank you, I'm looking forward to it.