How to reproduce:
1. Open any Unity project
2. Open the Package Manager window
3. Wait until the packages are loaded
5. If "all packages" is the upper left menu selection, skip to step 11, otherwise go to step 6
6. Change the upper left menu selection to "all packages"
7. Close Unity
8. Open any Unity project
9. Open the Package Manager window
10. Wait until the packages are loaded
11. Attempt to select anything in the Package Manager

Expected result: Package Manager window is responsive and responds to clicks/scrolling
Actual result: Package Manager window is unresponsive and does not respond to clicks/scrolling

Reproducible with: 2019.3.0a10
Not reproducible with: 2018.4.4f1, 2019.1.11f1, 2019.2.0b10, 2019.3.0a9
Could not test with: 2017.4.30f1(Package Manager window not yet implemented)

Notes:
Closing and re-opening the window fixes this issue
If the issue does not reproduce reset the Editor layout to 'Default' and repeat steps 1-4
Originally found on MacOS, Reproduced on Windows.