fix: sync top group if necessary even when dropdown is opened

[ugur-vaadin]

Description

This PR makes sure the topGroup items are always in sync with the selected items even when the dropdown is opened. The topGroup items are used to hold the references to the items that will be displayed on top when selectedItemsOnTop is set true. This fix does not change the intended behaviour of retaining the visible item order when the selected items are updated while the dropdown is opened.

Fixes #7161

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/contributing/pr
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud