Steps to reproduce:

1. Open any Unity project
2. Open 'Build Settings' window (or any other window)
3. Try to close it by clicking CMD+W or File/Close option
4. Notice it doesn't close with the key shortcut and the Close option is gone from File menu

Other editor windows, like Scene View, Inspector, Project, etc. also fail to close with CMD+W like they used to (but these only used to close when they aren't docked to anything)

Reproduced with: 2018.3.0b1
Not reproducible with: 2018.3.0a11