Add SelectionOrder DataAdapter decorator and related options

[devmonkey22]

Add SelectionOrder DataAdapter decorator and related options as a proposal to resolve #3106 with some limitations for manual selection changes (as noted).

• This class has to keep both display and selection ordering for all options in order to manage selection order, while maintaining the default results/query ordering (when sorter is not used, etc).
• Notable but unfortunate limitation: Setting the control value using jQuery.val([...]) with multiple items or manually toggling option selected (without a change event) still cannot preserve the value ordering.
  • The partial workaround for this is to manually set your selected options with the data-selection-order attribute before setting val() if possible.
  • It may also be possible to enhance the decorator to watch for manual changes via MutationObserver. TBD.

New options:

• keepSelectionOrder
• trackManualSelectionOrder

Update docs.
Add tests.

This pull request includes a

• Bug fix
• New feature
• Translation

[devmonkey22]

So I played with MutationObserver a little more. Unfortunately it doesn't fire for property changes (option.selected etc), so there is still no consistent way to watch for jQuery.val sets (without modifying jQuery.valHooks which I think is pretty discouraged. So this is still a limitation of the solution without a different workaround like:

• Setting option selections individually with a change event each like

  select2/tests/data/selectionOrder-tests.js

  Line 555 in 5fbd45f

  $select.find('option[value=3]').prop('selected', true).trigger('change');

• Manually setting the data-selection-order attribute of each option before setting jQuery.val([...])

[devmonkey22]

I figured I'd throw this out here. This is a 6 year old problem with an imperfect solution, but I don't think there ever will be (without MutationObserver changes perhaps) given the constraints of using a real <select> and how HTML/JS behaves natively.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.