How to reproduce:
1. Create a new project
2. Add 'Addressables System' package in the Package Manager
3. Go to Window -> Asset Management -> Addressable Assets
4. Press 'Create Addressables Settings' and add some new groups (right-click -> Create new Group -> Packed Assets)
5. Minimize the window until the scroll bar appears
6. Right-click on an existing group or somewhere around them

Expected result: The menu should display the "Create New Group" selection
Actual result: No way to add a new Packed Assets Group

Reproducible: 2018.3.2f1, 2019.1.0a14, 2019.2.0a1
Can't reproduce: 2018.3.0a1 - 2018.3.0a4, 2019.1.0a1 - 2019.1.0a7 because of missing Window -> Asset Management -> 'Addressable Assets' selection

Notes:
- The Addressable Asset system was added in Unity 2018.2