Library module full view timing out

A qurious problem has started to occur after many years of use.

In the library module, if I click F to go into full screen view on an image. it times out after approx 10 secs and returns to grid view. I can find no appropriate settings in preferences or under the view menu so I'm puzzeled as to why this has started to happen.

As an extra twist I have now discovered that if I start an unsaved slide show it also stops after a similar time. If I manually restart it does cotinue from where it stopped.

LRc on Windows 10. Both on latest versions.

Any ideas anyone?

Martin