Describe your issue or idea

There's been a number of comments over the years about adding a multi-select library to core to improve the UX in a variety of ways. I'll link the related issues below and provide a few pullquotes:

• [UX] Remove multi-select list form elements from core by switching to Select2 #1800 | [UX] Remove multi-select list form elements from core by switching to Select2
• Replace UI autocomplete with Select2 #459 | Replace UI autocomplete with Select2
• [UX] Change tags field display to use 'tag blocks' when entering tags #2121 | [UX] Change tags field display to use 'tag blocks' when entering tags

Other ideas and existing issues that reference select2:

• Abandoned (incomplete?) port of the Drupal Select2 module may provide a place to start via Replace UI autocomplete with Select2 #459 (comment) :
  https://github.com/backdrop-contrib/select2

• Comment from @jenlampton on [UX] Remove multi-select list form elements from core by switching to Select2 #1800 (comment) RE: the SelectWoo fork that may be better from an accessibility standpoint:

think I'm also leaning towards Select2 in terms of both features and aesthetic. As an added bonus, we could also use Select2 to replace our ancient autocomplete JS code. Here's an issue for Drupal about doing that very thing: https://www.drupal.org/node/2346973 That issue looks stalled due to front-end performance on the autocomplete, and accessibility. There appears to be some accessibility improvements in the SelectWoo fork.

• Comment from myself on Modules page tag filters [UX] Add "tags" to the modules page and in .info files #37 (comment)

(And as the select list of tags grows and becomes more unwieldy, hopefully it will turn into a select2 list allowing typing/autocomplete when/if select2 gets into core.)

• Comment by @serundeputy on [UX] move the clear log messages button to bottom of dblog messages page #2353 (comment) RE: 'Recent log messages' page filters

I think this improves the UX quite a bit, and we should hold off on the second bullet and address that UX issue when we get the select2 library in core

• Comment by @klonos on [UX] timezone drop-down should group timezones by continent #2928 (comment) RE: Timezone drop-down

(Triaged as possible accessibility issue on 08/17/2921. This one only seems to have come up because of possible accessibility issue with the suggested libraries, please add the [A11Y] tag if you think that this is an accessibility issue.)