After this question came up again (see comments), I did some digging around on different sites and Metas as well as some testing, and realized that the current behavior for tag synonyms is confusing and inconsistent, even within the same site.
I used the tag list and the tag box on a new question on
- Arqade using "cs-global-offensive" = "counter-strike-go"
- META SE using "on-topic-definition" = "scope"
- RPG SE using "scenario-authoring" = "adventure-writing"
To see what shows up and what does not. I will mention only synonyms for the Master tag, even if they differ from the one I tried.
- "cs-": List shows "cs-global-offensive" and "csgo", Question shows nothing
- "cs-g": List shows "csgo", Question shows nothing
- "global": List shows "global-offensive", Question shows nothing
- "cs-global-offensive": List shows "cs-global-offensive", Questions shows Master
- "scenario-": List shows "scenario-authoring", Question shows nothing
- "scenario-a": List shows nothing, Question shows nothing
- "authoring": List shows nothing, Question shows nothing
- "scenario-authoring": List shows "scenario-authoring", Questions shows Master
- "on-": List shows many matches, Question shows 6 matches (maximum)
- "on-t": List shows nothing, Question shows Master
- "topic": List shows nothing, Question shows Master
- "on-topic-discussion": List shows nothing, Question shows Master
Analyzing this, Arqade and RPG behave the same, partials matching the beginning of the synonym show the synonym in the tag list, the Question tag box needs the fully correct synonym to display anything at all. Other sides (Judaism and Math) seem to share this behavior as far as I can tell from the related topics I will link in the comments.
META SE seems to not show synonyms in the tag-list at all, and any partial match seems to show the master (if not too many matches)
I think, any partial match of a synonym should show the master in both the tag list and the question tag box. There's no need to show something to a user that will be automatically replaced anyway (Though There might be a non-obvious reason to show synonyms in the Tag list as well).
None of the sites I looked at have that behavior, but partial synonym-matching works for the question tag box on META SE, so we know it can be done, at least for the question tag box.
Note: There are the following statements by Anna Lear, concerning the behavior of the Tag lists.
This is currently by design, but we'll see about modifying the SQL that powers this search to include partial matches in the middle of the tag for synonyms. – Anna Lear♦ Jul 29 '13 at 17:02
We still think this is a good idea, but unfortunately it's not going to happen any time soon. – Anna Lear♦ Oct 28 '13 at 20:25
Should that still be valid, disregard the parts of this question concerning the tag list, and focus on bringing partial tag matching to question tag boxes all over SE, not just META (and potentially select others).