How to reproduce:
1. Download, extract and open attached project (packmanissue.zip) or create a new one
2. Open Package Manager Window
3. Click on any package (for example Ads)
4. Click 'See all versions'
5. Scroll back and forth to view different package versions

Expected results: A dropdown occured after clicking 'See all versions' with different package versions
Actual results: An invisible dropdown occurs after clicking 'See all versions', overlapping with other UI elements and buttons for different package versions are invisible and in some cases offset.

Reproducible with: 2019.1.0a12

Note: Does not affect earlier versions due to this UI being introduced in 2019.1.0a11