Steps to reproduce:
1. Create a new project or open an existing one
2. Open Package Manager (Window -> Package Manager)
3. Dock the Package Manager window to the left side of the screen
4. Click the plus icon at the top left to add a package

Expected results: The Add a package menu appears
Actual results: The icon is clicked and highlighted, but no menu appears

Reproducible with: 2019.3.0a1, 2019.4.34f1, 2020.2.0b8
Not reproducible with: 2020.2.0b9, 2020.3.21f1

Notes:
- Not reproducible on Windows, likely has something to do with Mac feature allowing to doc windows in display, where Windows doesn't have that