Enable full keyboard access, and Tab to the Settings button. It will disappear (becomes hidden). Press Spacebar or Return before moving focus to something else.

The hidden button still remains the document.activeElement, so subsequent keypresses (and clicks) will target the button, even though it is no longer focusable or clickable in the conventional sense. I believe this is either a browser bug or a spec bug.

Some focusable content before the button.

Focus hides me, but Spacebar and Return keypresses still activate me, even when hidden.

Some focusable content after the button