Comment 4 for bug 1331475

Can we display an overlay with an "x" button that is hidden/shown when the chrome is hidden/shown and toggles when tapping on the window?

Seems the only gesture that could work would be a bottom edge swipe as the other edges already perform actions.

As a fallback, we should exit fullscreen mode when browser moves to the background, such that when it's brought back to foreground the window is no longer fullscreen.