Why do some question pages have the first tag in the document title but others don't?

Question

Compare the document title (typically shown on the tab for browser with tabs) for these questions:

• programming - How can I use NestWhileList instead of While in this case?
• equation solving - Elimination returning true instead of correct answer
• Plotting difference between a function and its inverse function

The first two have a title that starts with one of the tags the question has (respectively "programming - ", and "equation solving - "). The last question is tagged too, but no tags appear in the title. Does anyone know why?

I like removing those starting tags from titles with a userscript and noticed this because my script failed by using the naive assumption that the tag is always present. What are the rules for how these titles are formed?

Answer 1

I have noticed this too but I have never explored it. I think that the page title tag is added unless it or one of its synonyms already appears (as a separate word) in the title text. I shall look for couter-examples to this hypothesis, as well as possible confirmation on the main https://meta.stackexchange.com/ site.

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Here is apparent confirmation of my hypothesis:

Should we stop adding the most popular tag to the HTML title on every SE site?

If you work the tag into the title "organically", it won't get prefixed. So, "How do I do X in Python?" won't have "Python - " prepended to it. So if you're just looking to put something more important near the start of the title, use this method to enforce it. – Shog9