Button accelerators not visible (but working)

I have two laptops - both running Linux Mint 18.2, Xfce 64bit, both with LibreOffice Unless I am completely missing something I have set the LO Tools>Options the same for both computers.

When I set them both up I noticed that in Calc documents with buttons (push buttons or options buttons) the underlined accelerators were not visible. (But they did actually work - I noticed that because I had predetermined the accelerator with the tilde~). This was the same for both laptops

Later I must have altered some setting somewhere (in LO or Linux I know not which!) because now one of my computers shows the accelerators perfectly well but the other doesn’t.

Can anybody tell me where the setting is?

UI was changed to hide them, and use to toggle the assigned mnemonic visible.

L-alt toggle doesn’t seem to do anything.
I think it is something to do with Xcfe settings.

****** STOP PRESS ********
With my various fiddling about with settings I can now see the accelerators on buttons on BOTH laptops.
What did I do?
I’m not 100% sure but I think it was to set Menu>Settings manger > Desktop settings - set dropdown as Xfwm4+compositing THEN DO A RE-BOOT

Hope this helps somebody else if they experience the same problem.