Full title: 'Add Component' button jumps on top of other properties in the Inspector window when resizing the Inspector window and previously inspected object had a scrollbar for the Inspector window when a custom expandable UI Group element is used

How to reproduce:
1. Open the attached 'Test.zip' project
2. In the Hierarchy window select 'Volume Spots'
3. Adjust the Inspector window height slightly so it would have a scrollbar
4. In the Hierarchy window select 'Volume Mesh'
5. Resize the Inspector window's height from bottom to top slowly (till 'Add Component' button jumps)

Expected result: the 'Add Component' button does not jump on top of other properties in the Inspector window
Actual result: 'Add Component' button jumps on top of other properties in the Inspector window

Reproducible with: 2019.1.0a10, 2019.3.9f1, 2020.1.0b4, 2020.2.0a5
Not reproducible with: 2017.4.39f1, 2018.4.20f1, 2019.1.0a9