Reproduction steps:
1. In Windows Taskbar settings set your "Taskbar location on screen" to Top
2. Download and open the attached project 'taskbar.zip'
3. Make sure Editor is not Maximized
4. Observe

Expected result: Unity window is placed under Taskbar
Actual result: Unity window is placed into Taskbar

Reproducible with: 2019.4.17f1, 2020.1.17f1, 2020.2.1f1, 2021.1.0b3, 2021.2.0a2
Not reproducible with: 2018.4.30f1

Notes:
- It seems that the Editor gets put in the right place if it's minimized and then opened again, yet not always.
- Trying to recreate a project on the same machine by having a taskbar on the bottom then exiting the project, changing the taskbar to be on top, and reopening the project doesn't reproduce the issue
- Issue can be reproduced if the project was saved in macOS or Windows environments before (both projects attached)