Thank you. It's just CSS and the :hover pseudo-selector, the popover effect is done using CSS keyframe animations. Fun geek fact: The Mac OS style close/minimi

Thank you. It's just CSS and the :hover pseudo-selector, the popover effect is done using CSS keyframe animations. Fun geek fact: The Mac OS style close/minimize/expand buttons on the code editor are written in CSS, they're not images. A bit of a pointless exercise but a fun little easter egg.