How to reproduce:
1. Extract the attached "1268917 repro.zip" archive directly into your drive
2. Open the "1268917 repro" project contained in the "_________________________________________117 character name__________________________________________________________" folder
3. Open the Package Manager window from Window > Package Manager
4. Select Timeline package
5. Select Cinemachine package

Expected results: Timeline package gets unselected
Actual results: Timeline package is stuck as selected

Reproducible with: 2019.3.0a10, 2019.4.7f1, 2020.1.1f1, 2020.2.0a20
Not reproducible with: 2018.4.25f1, 2019.3.0a9

Notes:
-In 2019.3 alpha versions, it's difficult to reproduce the issue because the Package Manager window is unclickable most of the time due to Cinemachine having a path that's too long. Resetting packages, restarting the Editor, and installing Cinemachine again helps to overcome this.
-Steps 4 and 5 can be repeated multiple times to get multiple packages stuck as selected