Years ago, they changed the behavior of the green button to be "fullscreen into a separate space." As someone who never uses spaces, this is never what I want.
You can escape it by moving your cursor to the top edge of the screen and clicking the green button on the titlebar that appears to exit fullscreen.
You can escape it by moving your cursor to the top edge of the screen and clicking the green button on the titlebar that appears to exit fullscreen.