Replace UI autocomplete with Select2

[klonos]

This is the same as D8 issue https://www.drupal.org/node/2346973.

PS: Setting it to 1.1.0 because I know we are rushing to get 1.0.0 out soon. Feel free to push even further if you feel it's right.

(Triaged as possible accessibility issue on 08/17/2921. This one came up in search because of possible accessibility problems with the solution so no 'A11Y' tag today. Please add the 'A11Y' tag if you think it belongs here.)

[klonos]

...and since this requires a 3rd party library, I'd like to give #159 a "push".

[quicksketch]

We're actually not even using jQuery UI autocomplete even now, we're still using the autocomplete.js file that was written back in D5 days. This element has served us well considering it's age, but it has shortcomings (a lack of hidden value for submission, no keyboard arrow support).

Wow, the spreadsheet analysis of libraries at https://docs.google.com/spreadsheets/d/1vifOGL_4wgOl6sy2Os59k24saIHvwFFQoi5brH5Uv28/edit#gid=0 is pretty incredible.

A 1.1.0 target sounds great. This would also require styling of our select lists in Seven (we have design resources now working on that problem) for consistency with the Select2 styling. Right now the two right next each other can be pretty jarring.

[klonos]

Wow, the spreadsheet analysis of libraries ... is pretty incredible

Indeed! And from the huge list of features, one can see that Select2 is the clear winner.

...we're still using the autocomplete.js file that was written back in D5 days.

Perhaps a 1.0.x target then?

[quicksketch]

Well this hit a pretty serious snag when Select2 removed support for textfields (and thus our primary use-case), see https://www.drupal.org/node/2346973#comment-9518139. But it looks like support has been restored in the more recent release. We won't have time to implement this before 1.1.0 (due next week) so I'm moving this back into the general 1.x pool.

[MrHaroldA]

Here's a Drupal 7 implementation we could investigate/port ...

I made a quick port of the D7 Select2 project above (stable release of Dec. 2015), and I can confirm that a lot of things have changed since May 2015 : with select2 plugin version 3.5.4, on a taxonomy autocomplete field :

the input textfield is preserved, the select2 plugin item list is populated by ajax only by request, so it would be scalable…

A backdrop initial port is at https://github.com/gifad/select2 , if any one wants to check deeper
(warning : my work around "taxonomy pre fork change notices" is not fully tested)