Steps to reproduce:
1. Open a new Unity project
2. If on Mac - make sure to have 2 monitors side by side, and Unity is opened on the right one (not the left one)
3. Open Package Manager and place it's window on the left side of the screen (works both docked and not docked)
4. Click the + sign to open "Add package" pop-up

Expected result: "Add package" pop-up appears on the right screen in Package Manager's window and is visible
Actual result: "Add package" pop-up is not visible, though it actually exists on the left monitor and can be interacted with but is not visible

Reproducible with: 2019.4.0f1, 2019.4.11f1, 2020.1.6f1, 2020.2.0b5
Not reproducible: 2018.4.27f1 (because Package Manager had different UI)

Notes:
-For Mac it's important how 2 displays are placed together in order to reproduce this issue
-When on Windows, it's easiest to reproduce when Unity is maximised, though it's not required. Display's positioning does not have an effect