Your example looks as if you called swapbuffers 8, \@buffer.

You're exactly right! I changed the array, but not the number. I apologise. And thank you.

It also seems to be very similar to graff's proposal 1118234 that I do not quite understand still.

I "understand" his logic; but coding it defeated me before I needed sleep.

And I now appear to have two working solutions, so I may never go back to it...