The Google bug has now begun to affect me and I can provoke it and get rid of
it at will. Control F5 (Refresh on Windows) will provoke it if not already
visible and F5 alone will get rid of it.
I have yet to notice any other pattern. Single pictures appear to be always affected and four in a row or a second line of pictures isn’t despite all being set to obey the same instruction. it will now be possible to experiment with alternative instructions but as the existing one is quite obviously correct it may well mess up browsers that correctly observe the standard.
Thanks to those who reminded me that I had only mentioned the F5 trick in email responses to affected readers and not on the blog.