Update December 2016: This extension is obsolete. This feature made it into Hacker News natively, finally!
One thing I miss from Hacker News is the ability to collapse comment trees. Long threads can be quite hard to read and it is easy to lose your place. I got used to this feature on reddit and wanted to bring it over.
Here are a Safari extension and a userscript, whichever you prefer. Both add a button to every comment, click it to hide or show child comments.
Before starting, I looked around to see if this had been done. I found this old bookmarklet by Alexander Kirk. It had a few bugs and did not quite work as I intended. For one thing, it is a bookmarklet, so you have to manually activate it on every thread you visit. Extensions and userscripts would work automatically.
Instead of starting from scratch, I used Alexander’s bookmarklet as a starting point. I still kept a lot of the old code so it may be a bit unruly.
I’m really satisfied with how this feature turned out.
The code is on Github.