EuiComboBox with single selection and custom option should not show a list

[thompsongl]

Summary

Fixes #1872, in which an EuiComboBox with singleSelection and onAddCustomOption re-opens the options list after adding a custom option even though no more selections can be made.

If the custom option does not show in the options list, the user now must clear the search/selection before the options list will open.

@cchaos, let me know if I need to adjust experience further to get at what you intended

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs
- [ ] Documentation examples were added

• A changelog entry exists and is marked appropriately

- [ ] This was checked for breaking changes and labeled appropriately
- [ ] Jest tests were updated or added to match the most common scenarios
- [ ] This was checked against keyboard-only and screenreader scenarios
- [ ] This required updates to Framer X components

[chandlerprall]

I'll code review after Caroline has done a UX pass.

[cchaos]

Can/Should we also disabled the dropdown arrow button? Seems like it's a bit misleading that clicking the arrow no longer does anything. The alternative would be to allow forcing the dropdown to show when clicking the arrow, but not when just entering into the input. The latter may be more ideal but also more complicated.

[thompsongl]

@cchaos Seems reasonable. I'll try the 2nd option.

[thompsongl]

Updated to have the arrow button open the list regardless of single selection custom option state

[cchaos]

WFM! TY!

One thing I did notice is that in FF, combo box dropdowns won't close when the arrow is clicked.

Might be related to #1385

Don't forget a CL, and maybe a test would be helpful.

[thompsongl]

^ That's been a thing in FireFox, I guess (reproducable in the prod docs).

Specifically, it has to do with the focusout event listener and its relatedTarget value, which may or may not be what #1385 is getting at. I'll look a bit more, but likely will open a new issue for what you see.

[thompsongl]

Updated CL and created #1885 to fix the Firefox issue @cchaos saw (new PR as it may require additional discussion not related to this diff)

[chandlerprall]

changes LGTM, pulled & tested docs locally