OK, think I solved the problem, but this sort of makes me a bit mad that Xara doesn't have better quality control... is this just a new v15 problem? I've spent a whole day trying to figure out what the problem was and taking up your time here too.
Anyway... This morning I opened a different gallery page from the Wildside template and got a mobile and main window.
I set the auto fit to width only and changed the page options from 760 to 960 pix

I then dragged four photos in to the main window, sized them to fit, selected them all and added a mouse over and previewed.. OK

Selected each one and added text to the title and checked for popup and for options selected dim,shadow,title with gallery options set to browser controls and bottom film strip
Previewed and all is working forward and back with browser control and keyboard keys, slide show works and film strip shows the four images.

Selected them all and pasted them into the mobile window and sized them to fit. No variants are shared on any of them.

Previewed again and all is working fine in the full size and the mobile size

HERE IS THE BUG: In the main window I selected all 4 and added a shadow to them.
Previewed and now I have the same problem I had before. Only the left browser arrow works, there are double images of each one in the film strip and the keyboard only works with the left and up arrow keys
While in preview mode I narrowed it down to the mobile view and that one works fine (I had not added shadows to the thumbs in the mobile window)

I did a Undo several times to remove the shadows in the main window and previewed again and all is working fine again.

Repeated this several times and apparently adding shadows to the thumbs screws up the popup functions cycling.

Someone else please confirm my finding. I'm going to post this over on the Magix form as well, although I didn't get one comment over there.