Quitting games in losing possition

What if they just need to fix something with their computer?

If they go offline at all, can you claim a win immediately?

#11 Not immediately, but after a certain time.
The timer is more forgiving in slower time controls. This is why players with poor internet connection are not recommended to play ultra, hyper, bullet or super blitz.
The OP is obviously pointing out another kind of (abusive) behavior. They are not talking about people losing connection, but about sore losers who are staying online, sitting there, starring at their inevitable loss and trying to soften it by saying: "you will win, but I am going to make you wait!" :<
Please don't resign against them. Report them in case there was no automatic warning after the game in the chat. Block them.
Sooner or later they will either get temporary banned from playing or get blocked by everyone.
Just relax and let the negative people consume themselves :-)

@TrainingOTB Totally agree, but I wouldn't want to give up an opportunity to make my opponent waste time, so I would more readily start another game in tandem.

Assuming this is no problem in bullet and a minor problem in blitz this is probably about rapid and classical chess.

There is always something to do in my household which can be done under 5 minutes. If this happens I fill up my cup of tea, I look into the email, ... If the game is finished I block the opponent. There are days, when this happends two or three times and weeks when this happens not at all.

So what?

Ahh tea

Ok if your opponent waits for their clock to run out after you push them into a losing position, then just wait off the game and let the lichess algorithm catch them. I don't know about waiting though, that's your choice. I mean... I normally wait it off, including a time when I was playing a 180+0 classical game as anonymous and I waited for 3 hours (kept my phone open for that long) and I won... and then I realized how stupid of an idea that is. But if it does happen in a classical game, then I'd suggest resign and report.

@danielbest , @ShootingStar34 I think that the problem is that the opponent still was online but didn’t make a move.

These toxic players are literally everywhere. you can troll them, (If you have the time), by giving them plus 15 secs everytime they get close to there time running out, and then show them whose boss 😉

@MoonRising lol wow or u could delay a forced checkmate and imagine them panic but if you don't want to risk it then I don't recommend doing it