Steps to reproduce:

1. Open a Unity Project on a computer with a multi-monitor setup.
2. Undock one of the editor windows (e.g. Game window) so it is floating on your main window.
3. Disconnect one of your additional monitors

Actual results: The floating window will disappear and become completely inaccessible. Trying to open the window again (e.g. through Window > General > Game for the Game window) will do nothing. You need to reconnect the additional monitor for it to become accessible again.

Expected results: The floating window will remain.

Reproducible with versions: 

• 6000.0.10f1
• 2023.2.20f1
• 2023.2.0a14

Not reproducible with versions: 

• 2023.2.0a13
• 2022.3.36f1
• 2021.3.40f1

Can’t test with versions: 

Tested on (OS): Windows 11

Notes:

• This bug is not reproducible all the time. If the bug does not occur try:

** Re-dock the window and then re-do the repro steps from step 2.
** Re-dock the window. Then restart the editor and re-do the repro steps from step 2.
* Try with another editor window (e.g. if you used the Game window, then try using the Animator or Scene window).
There is a less severe version of this bug that affects the Editor Windows even on earlier Unity versions than this bug. That issue has been linked to this one.
** UUM-75313 Floating Editor Windows resize to smallest size when disconnecting monitor on multi-monitor setup - Unity JIRA (unity3d.com)