In Lightroom 2015.7 if I press '\' to toggle before and after view the grey background colour changes shade. This only appears to be the case when 'darker grey' is selected. This change is distracting when trying to make a comparison!
Hi Rikk, I'm using Windows 10 Pro on a Surface Pro 4.
I first noticed it when editing a portrait image so a lot more of the border was visible. Then hitting '\' to flick between the before and after view causes the shade of grey to change. Strange! Just tried it again, still doing it.