The Windows, Mac, and Linux clients now present a 'Close Game' button in the bottom right when you activate Windowed Fullscreen mode. Additionally hitting 'Escape' will bring up a menu with a close option. I didn't mean to hold you hostage, browser games just don't normally need a "Close Game" button!
This also adds Ctrl +, Ctrl -, and Ctrl 0 (or CMD on Mac) hotkeys to allow zooming the entire interface in/out or resetting it with Ctrl 0.