When Assets are added multiple times until the scrollbar is appeared then on using Add() & Remove(-) Buttons they don't show intended behavior in Search Index Manager window.

Steps to repro:
1. Create a New Project
2. Click on Search Icon besides collab Icon
3. Enter "Index Manager" in the Global Search window and Select
4. Search Index Manager > Add() Assets multiple times until scrollbar is appear
5. Click on Add() & Remove(-) buttons

Actual Result:
Add() & Remove(-) buttons doesn't work with intended behavior in Search Index Manager window

Expected Result:
Using Add(+) - Assets should be added in Search Index Manager window
Using Remove(-) - Assets should be removed in Search Index Manager

Reproducible in:
2021.1.0a2, 2021.1.0a1

Environment:
Occurring on Windows 10 & macOS 10.15.

Note: Global Search option in Editor is introduced in 2021.1.0a1