Use standard dropdowns for combobox

[timja]

Fixes jenkinsci/design-library-plugin#13

Unfortunately can't delete https://github.com/jenkinsci/jenkins/blob/master/war/src/main/webapp/scripts/combobox.js as editableComboBox still uses it, #9461 removes the last usage in core though.

This improves the usability and accessibility of the control by adding styling when doing keyboard navigation, and makes the code a lot simpler and easier to follow (last one used very old JavaScript design patterns and was hard to understand)

I haven't added support for allowMultipleValues as I couldn't find any usages of it.

Testing done

Tested with the design library example http://localhost:8080/jenkins/design-library/Inputs/

Keyboard navigation, tab and enter on item
Clicking the item
Tabbing away before selecting an item
Editing items and making sure they show up

Unlike autocomplete, combobox shows items immediately so tested some special handling for that

I've also tested with #9461 (which was motivated by this PR)

Proposed changelog entries

• Modernise combobox component

Proposed upgrade guidelines

N/A

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@mention

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[timja]

occurs when navigating between potential text left and right without selecting an item (doesn't cause an issue just stacktraces in the console)

e.g. type North, then navigate left on the keyboard

[zbynek]

Looks like quite a bit of code was copied from #9453, some parts were improved here (e.g. I missed the focusout listener). Maybe it would make sense to land this PR first, then refactor mine to avoid the duplication and make use of the improvements you've done here? Or would you prefer the other order?

[timja]

Looks like quite a bit of code was copied from #9453, some parts were improved here (e.g. I missed the focusout listener). Maybe it would make sense to land this PR first, then refactor mine to avoid the duplication and make use of the improvements you've done here? Or would you prefer the other order?

I don't mind either way, I've just noticed that the Dynamic combobox example is broken though so will convert this back to draft and take another look later

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[timja]

Retested all with the examples on http://localhost:8080/jenkins/design-library/Inputs/

I've refactored out some common code as well.

[zbynek]

Looks good!

[timja]

@janfaracik you able to check this please?

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!