How to reproduce:
1. Open the attached "1306562_Repro" project
2. In the Project window make sure it's "One Column Layout" style and press on an empty scroll bar part and take not how much it scrolls
3. Open the Package Manager window (Window->Package Manager)
4. In the Package Manager select it to show all the packages (at the top of the Package Manager press on the Packages drop-down menu and select "Unity Registry")
5. Press on an empty scroll bar spot and take note of how much it scrolls

Expected results: Both scrolls bars scroll the same amount
Actual results: In the Project window the scroll bar scrolls by a whole "page" (only one previously seen item will stay visible in the list) while in the Package Manager it scrolls only by ~3 items

Reproducible with: 2018.4.31f1, 2019.4.18f1, 2020.1.17f1, 2020.2.2f1, 2021.1.0b3, 2021.2.0a2

Notes:
- The Package Manager is the only place where the scroll bar scrolls by only ~3 items