`clx` is written in Go using Bubble Tea[1]. You can read the comment section or the linked article in reader mode in the pager `less`.
Using `lesskey` to add custom keybindings, the replies can be collapsed and expanded in real-time (but not individual replies, only all replies at once). Behind the scenes I am appending invisible unicode characters to each line so that I can use the custom keybindings to filter them out. The same technique is used to allow for jumping between top-level comments.
I spent a lot of time thinking about syntax highlighting and finding relevant bits to highlight while also not going overboard with colors. The end result is highlighting of things I find useful for providing context in the comment section, like indicating parent poster and original poster, coloring references, coloring indentations as well as formatting YC startups.
Other quality of life features include adding submissions as favorites (it is stored as a pretty-printed json so you can check it into your vcs to allow for readable diffs). Submissions are marked as read and new comments are indicated with a bullet point.