@fregante
PR updated after review. Couple of notes:

• Repo-wide t works only from Issues & Pull Requests tab. This is because only those 2 are inside the pjax-container element given by Github. Since now we can be sure that FF is always loaded via pjax, I'm using a normal variable as buffer like you said
• For text restoration, tried using Github's inbuilt js-session-resumable class. Unfortunately, adding the class didn't work out; probably the code that handles it runs before that, like you had hinted. So went with storing in sessionStorage.
• Should have been 3 PRs, but initially didn't have the idea they were gonna be 3 different features. Hence single PR 😄

Looks good so far! I’m not sure when I’ll get around testing the code though

The interesting part is that GitHub already has a js-pjax-restore-captured-input class on the field but apparently it doesn't work. I guess/hope preserve-file-finder-term will have a short life.

I hope to see this merged soon, it's only missing descriptions and screenshots.

Maybe you can find an elegant way to say "if responsive view, append to body and add the sr-only class instead, so it works even if invisible.

This can be done in a later PR, perhaps, so this one is merged asap.

@fregante Yes, I'll look into the description & screenshots over the weekend.

I never thought this'd see the day!