Steps to reproduce:
1. Create a new Unity project
2. Undock any window from the Editor and move it onto the Desktop (ex: Console window)
3. Maximize the window to fullscreen by clicking on the green button in the toolbar
4. Click on the green button again to exit fullscreen

Expected Result: The window exits fullscreen
Actual Result: The window does not exit fullscreen

Reproducible with: 2020.2.0b2, 2021.1.0a1, 2021.0a6
Not Reproducible with: 2018.4.29f1, 2019.4.15f1, 2020.1.14f1, 2020.2.0a21, 2020.2.0b1

Note:
The button to exit fullscreen is different in Unity Editor Window than in other apps like Slack. The attached images show the difference