In the Player Settings -> Resolution & Presentation, if user chooses to launch Player into Exclusive Fullscreen mode without checking option "Allow Fullscreen Switch", the Player launches into a window with title bar, but on top of all other windows, menu bar and dock. But it is in the same space where it is launched, instead of creating its own space.

Repro:
1. Create or use an existing project on 2018.3.0a1
2. Go to Player Settings -> Resolution & Presentation.
Set "Fullscreen Mode" to "Exclusive Fullscreen"
Set "Allow Fullscreen Switch" to "Checked"
3. Build the Player and launch
4. Change "Allow Fullscreen Switch" to "Unchecked"
5. Build and launch the Player
6. Compare the difference.

Project download: https://oc.unity3d.com/index.php/s/b67c5oh65wUg7zl

Reproduced with: 2018.2.0b11
Not reproducible: 2018.2.0b10