For me it's the "Stop responding" button. Sometimes I'll neglect something in my prompt, such as the fact that I'm stuck on ES5 javascript in my job (ServiceNow). It'll spit out ES6+ with let declarations or something like that, and I have to go back and qualify my limitations. So I click stop responding. What used to happen was that it would stop and allow for additional prompting. Now it's just like a client side trick. It hides the output but the server is still returning shit in the background, so if I try to re-prompt or add context it finishes what it was originally saying first, then tacks the new answer onto the old one without pause, separation, or human readable formatting that would indicate that there is a new output. It's an awful experience.

I've been using perplexity.ai but my company thinks its agreements will stop Microsoft from training their AIs on our proprietary data, so I have to be more careful with perplexity than Copilot.