Sorry, all fixed. I only just noticed the "Screen with active window" is the default for the screen input on the Fullscreen node. So the fullscreen is being applied to the active window. All works if I just extract the correct screen and feed that to the Fullscreen node, I attach the fixed composition.
Following up from my last post, I am getting unexpected behaviour when going fullscreen on multiple displays. Seems like a bug to me.
In the attached I have my screens layout, namely one laptop and two external displays.
There is a minimal composition (archive.zip) that illustrates the problem.
There are photographs of the result, one before the periodic event for fullscreen, and one afterwards.
Thanks for the offer, but don't worry about it for now. I'll test later this afternoon or tomorrow and report back. On site in the dome shown in the original post was not the right place to debug things while the client was watching, I'm sure I'll be able to isolate the issue in the more relaxed home environment. :-)
I will check again later today with my two external LCD displays.
But I can confirm, I had no trouble with the internal and one external display from my laptop, the issue only occurred with two external displays.