Editor.Instance.Windows.GameWin.IsDocked seems to be always true for me.

Should be very easy to reproduce. Just print it to the debug console.

What I have tested:

1. Game Window is undocked
2. Enter Game Mode
3. IsDocked -> true
4. Leave Game Mode
5. Dock Game Window
6. Enter Game Mode
7. IsDocked -> true
8. undock Game Window while in Game Mode
9. IsDocked -> true