Repro steps:
1. Open the attached project "1253664-handtool.zip"
2. Open the scene Assets/Scenes/SampleScene.unity and make sure that the Scene View tab is visible
3. Select the Hand Tool at the top of the Editor
4. While holding Alt+Left mouse button, try looking around the Scene using arrow keys
5. Release the Alt and Left mouse button

Expected results: it is possible to look around the Scene while holding Left, Middle or Right mouse button, the Eye tool icon changes back to the Hand tool icon
Actual results: it is only possible to look around the Scene using the arrow keys, it is impossible to choose any other tool; in some versions, the Hand tool gets stuck with the Eye tool icon

Reproducible with: 2018.4.0f1, 2018.4.23f1, 2019.3.0a1, 2019.3.16f1, 2019.4.0f1, 2020.1.0a1, 2020.1.0b13, 2020.2.0a1, 2020.2.0a15

Notes:
*The issue goes away when any of the mouse buttons are clicked while holding any of the arrow keys, entering and exiting Play Mode and toggling any one of the options in the Scene View tab (e.g. Scene lighting)