• Bug 48243 has been marked as a duplicate of this bug. ***

Update in light of T27909#1085035 :
Autocompletion could be based on the cached list of tags sorted by tag usage statistics. On selecting the tag input, the most used tags would be displayed first, then typing would narrow down the list based on the autocompletion method used (maybe the same as the one for searching, or a more simple one).

Change 175938 had a related patch set uploaded (by Gerrit Patch Uploader):
Add autocompletion for tag filters

https://gerrit.wikimedia.org/r/175938

What is specifically the difference to the tasks linked in "see also" (which have been resolved)?

Change 175938 abandoned by Bartosz Dziewoński:

[mediawiki/core@master] Add autocompletion for tag filters

Reason:

This feature has been implemented in another patch

https://gerrit.wikimedia.org/r/175938

In T23383#7751690, @matmarex wrote:

Indeed, I think this has been also resolved by the fix for T27909: Add a drop-down list for the tags in Special:Newpages, Special:Log and Special:Contributions.

@matmarex, can you confirm whether the implemented dropdown includes auto-complete / search-as-you-type functionality? I came across this from the Community Wishlist results page, and can see that the dropdown is already available e.g. on Special:Contributions, but typing in the text box doesn't seem to filter the content of the dropdown list.

Not really, it will only highlight exact matches (and it matches the internal value, rather than the displayed label, to make this more confusing). This isn't great, however, the widget displays the complete list of available tags (instead of only auto-completing after you start typing), so I think it's acceptable, and I don't really have more time to spend on this.

By the way this is just how our standard combobox widget works, not implemented specifically for the tags field. I filed a bug about it some time ago: T100876: Better "type to search" in ComboBoxWidget.