The most annoying page on mobile devices is the diff. One reason for that is
FS#2676, but another is that seeing two different version in tables side by side is difficult on small screens. (That's to do with
FS#2467.)
We already have a solution for that problem: Inline diff. But it's annoying to have to choose it again and again. One solution would be to always default to inline diff on mobile devices. But it's generally a good idea to keep whatever the user has chosen as a default. So, storing that choice in a cookie is a good idea. (Similar to my suggested solution to
FS#2438.)