[EuiComboBox] Improve virtualized list accessibility

[mgadewoll]

Description

Caused by this issue: elastic/kibana#206031

EuiComboBox currently does not appropriately handle its virtualized list to ensure expected output for screen readers.
When navigating the list and reaching the current visible end of the list, the next items are appended and previous items are removed, the screen reader output should not be affected by this.

Note

EUI plans to rebuild EuiComboBox with EuiSelectable (issue). This would solve this issue as EuiSelectable correctly handles the virtualized list updates. If the rebuild has happened already, this issue is obsolete and can be closed.

Unexpected behavior:

• the appended items being read out by the screen reader because the panel has an aria-live="assertive" applied
• the list size is updated
• the item position index is updated correctly

Screen.Recording.2025-01-10.at.12.11.02.mov

Expected behavior:

• the appended items are not read out
• the list size stays static and matches the available list item length
• the item position index does not update and matches the index in the options array

Screen.Recording.2025-01-10.at.12.12.03.mov