auiSelect2 z-index issue when used with 2 modals

Issue Summary

This is reproducible on Data Center: yes

The logic behind assigning a z-index to auiSelect2 is not working correctly. The drop-down list of a child modal window considers its parent not the modal window in which it was opened, that is - the very first open modal window. Because of this, the dropdown list is not visible.

In short, the parent modal has a z-index of 3000, the child has a z-index of 3100, and the dropdown has a z-index of 3002 (it should be 3102). The drop-down list considers the modal window to be the parent of the parent, and therefore the display does not work correctly.

This issue started in Jira 9.13 and does not appear in 9.12.

AUI Select2 uses version 3.5.x of Select2. In this version you can set the z-index with 

dropdownCss: { "z-index": "10000" } 

but this then causes errors in the UI with the Select2 dropdown not disappearing after use.

In version 4.x of https://select2.org/ it has option

dropdownParent: $('#child-modal-dialog') 

which would fix this issue.

This is also referenced in https://community.developer.atlassian.com/t/aui-dialog2-issue-with-select2-dropdown/77627

Steps to Reproduce

1. Create a button that appears on a modal in Jira, e.g. Create Issue
2. Create a modal that opens a new dialog
3. In the dialog create a new Select2

Expected Results

The Select2 dialog works as expected.

Actual Results

The Select2 drop down list does not appear in the view.

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available