On macOS, a JavaFX Window that is set as non-resizable can actually enter a new space in full screen mode via Mission Control, but that is unexpected and doesn't follow the macOS standards and specifications, as it implies it would be resized to fit the screen.

To reproduce the issue:

1. Run the attached application ("1. application.png")
2. Press Ctrl+Up, to open Mission Control ("2. Mission control.png")
4. Select, and drag the window and move it to the top Spaces bar, after the existing desktops, to its own "java" space ("3. java space.png"), which simply will make the window to go fullscreen in its own space, which is unexpected for a non-resizable window.
5. From the top Spaces bar, go to the java desktop (or without Mission Control, press Control+Right until you reach it). You will see the app resized in full screen ("4. full space.png"). Again, unexpected.
6.- Hover with the mouse to the top of the screen until the system menubar and the window controls show up ("5. system menubar.png"): the app can't exit full screen mode, unless moving it back from Spaces to an existing Desktop, or closing it directly.

This issue can be reproduced with any JavaFX version.