Before starting I tried searching but I couldn't find anything. Frequently I come across comments about people wanting this feature.
Here is the proof. [0]
> yes please! i need a "comment to follow" functionality on HN
So I built it. I built this for one user named swyx. I'm going to email him now letting him know I built it.
There have been quite a few of them over the years in varying degrees of complexity. I think the most well-known one is probably HN Replies. A simple Github search returns the following results.
HN Notifier
https://github.com/meanands/hn-notifier
Hacker New Telegram Bot
https://github.com/lawxls/HackerNews-Alerts-Bot
HN Reply Notifier
https://github.com/lotsofnoise/hn-reply-notifier
And there have been Show HNs on them as well.
Show HN: I built an open-source notification inbox widget for Hacker News
https://news.ycombinator.com/item?id=36373123
Show HN: HN Reader – Track your conversations across Hacker News
https://news.ycombinator.com/item?id=46240221
I (and probably a lot of other HN users) built my own bespoke HN suite that notifies me on replies, stories that I might be interested in, etc. It just sits quietly on a VPS and sends messages to me over Signal. Never open-sourced it because the code is a dire abomination.
Yours is probably the first Chrome side-panel variation that I've seen though.
There have definitely been a few over the years, especially if you include scenarios beyond just "get notified for replies". Some have been webapps, some have been browser plugins, etc.
Here's some discussion of one recent one:
https://news.ycombinator.com/item?id=21172406
And some more:
https://news.ycombinator.com/item?id=42814917
And HNChat, which went a little further:
https://news.ycombinator.com/item?id=12431724
That said, don't take this as criticism. It's clearly something that some HN users want. If you can make it stick, more power to you!