Impromptu slideshow has suddenly changed, how do I get caption to overlay onto the image?

LR Classic 15.0 impromptu slideshow (not the full scale generate version) has always show a full screen image with the caption overlaid ONTO the image. I think I may have set it up many years ago. Suddenly I’m getting the image surrounded by a large pale grey frame with the caption underneath. I have struggled and got somewhere near but I can’t see how to overlay the caption onto the image. Any ideas on how to do this and which option I’ve pressed to mess things up?

Thanks,

John