Are we not on "Hacker News"? Someone could make a simple browser extension that auths with your home instance and gives buttons to follow people on other instances.
It is technically impossible to be browsing user profiles on another server and then follow people with one click for this reason: People have multiple accounts. This is a feature to compartmentalize identities on the internet. You might have one account under your real name for people to find you, and you might have a different anonymous account for publicly posting political views that you don't want your fascist government or boss to associate to you.
Yeah, in that case, I think the status quo "works as designed". But I guess you could have the button trigger a drop down to select which account they want to follow from.
Couldn't the browser remember your home instance as an autocomplete "url" value[0]?
So you would click the "Add this person as a friend on my home instance" button on the third party page, and it would bring up a little text box which you could fill and submit with a couple of clicks, then it would redirect you to your instance with an agreed URL parameter, so that your instance would ask whether you really wanted to add them.
Assuming your instance remembers who you are with a cookie (and the third party instance can remember your home instance too), the process could be minimally onerous.
