The preview is shown while a link to a SE question or SE answer is hovered (the mouse cursor shouldn't move for at least 200ms). Also when hovering "# answers" block on the left in question list on SE sites.
To disable it press and hold any modifier key like Ctrl, Alt, Shift etc. TODO: add an option to reverse the behavior so that the preview is shown only while a modifier is held.
The preview can be "pinned" by quickly moving mouse cursor over it. TODO: add a pin button and a persistent mode for wide screens with the main page content shifted to the left.
Esc key closes the preview if it's focused via clicking or right after it's shown.
The preview may be resized by clicking its top border
Questions with answers have a shelf on the bottom with links to the answers. Clicking such an answer link replaces the preview. Clicking the preview title goes back to the question.
Inside the preview clicking a link to SE content will replace the preview, such links are underlined.
The data is cached internally for at least a minute for today's posts, more for the older ones. TODO: add a button to refresh the preview or do it automatically when the title/answer link is clicked.
Still in early development. Tested in Chrome 55 and Firefox 52.