You cannot scroll either the list view or the detail view of the Package Manager UI using the Mouse Scroll Wheel when a scroll bar is present. This is inconsistent with the rest of the editor.

Found with Package Manager UI version 1.9.8. It is not a regression as it has never worked.

1. Install Unity 2018.2+
2. Create a new project
3. Open the Package Manager and update the Package Manager UI to 1.9.8
4. Make the Package Manager window as small as possible
5. Select "All" in list view so that a scroll bar appears
6. Attempt to scroll up and down the list view using the Mouse Scroll Wheel
7. Select a package with a description long enough to make a scroll bar appear in the Detail View e.g. Package Manger UI
8. Attempt to scroll up and down the detail view using the Mouse Scroll Wheel