How to reproduce:
1. Create a new project
2. Open the Package Manager
3. Dock the window on a panel that is narrower than the default width of the Package Manager window
4. Try scrolling horizontally using a mouse or a trackpad

Expected results: Horizontal scrolling works
Actual results: Horizontal scrolling does not work

Reproducible in: 2023.3.0a2, 6000.0.4f1
Could not test in: 2021.3.39f1, 2022.3.31f1, 2023.2.0a23 (no horizontal scroll bar in Package Manager)

Reproduced on: Windows 11 Pro (23H2)
Not reproduced on: No other environment tested

Note: In version 2023.3.0a2 the horizontal scroll bar was introduced, but the scrolling is incredibly unintuitive (scrolling to the right, when the vertical scroll is at the bottom, and the opposite when the vertical scroll is at the top, horizontal scrolling can be activated by holding ALT or CTRL, but not SHIFT (as is standard), no scrolling using trackpad)