Reproduction steps:
1. Open the attached project "New Unity Project (7).zip"
2. In the Scene view press Alt and drag with right-click while using a tablet pen

Expected result: Alt+draggin with right-click rotates the Scene view when using a tablet pen
Actual result: Alt+draggin with right-click does not rotate the Scene view when using a tablet pen

Reproduces on: 2019.3.0a4, 2019.3.6f1, 2020.1.0b2, 2020.2.0a4
Doesn't reproduce on: 2017.4.38f1, 2018.4.19f1, 2019.3.0a3

Notes:
-Event.current.delta is (0,0) during drag
-After using Alt+draggin with left-click the issue is gone
-Does not reproduce on macOS