This SO question has several discussion comments going back and forth about whether the question belongs on SO or Meta.
The question is asking about how a specific feature of the site is implemented from a coding perspective (so that the OP might duplicate it in their own implementation), which can make it a legitimate programming question.
According to the Meta FAQ: "If your "question is about ... Stack Overflow / The core Stack Exchange engine", it should be asked here.
Here are some similar example questions on Meta:
- Textarea buttons and preview: How does it work?
- How does Stack Overflow do caching?
- How does Stack Overflow syntax highlight citations so well?
- How does SO enforce the daily vote limits?
Where do these questions belong?