What OS are you using?
Windows has special behavior when an app window is sized to the screen. For example, Windows may prevent the mouse from leaving the window. We can't stop that from happening. The "fullscreen" button sizes to the screen, so the special behavior occurs.
If you have a taskbar then "maximize" sizes Spine's window to the size of the screen minus the taskbar size. This doesn't trigger the special behavior because the window is smaller than the screen.
If your taskbar is set to auto hide or you have the window on a monitor with no taskbar, then "maximize" will make the window 1px smaller than the screen. This is the only way we can avoid the special behavior. You should never see the special behavior when you maximize.
