SE Preview on hover

The preview is shown while a compatible SE link is hovered.
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. Such "pinned" preview currently doesn't have a button to close it so to hide it simply move quickly over any other compatible link in the main document. ^{TODO: add pin/close buttons and a persistent mode for wide screens with the main page content shifted to the left.}
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.}

Alpha verion. Tested in Chrome 55 and Firefox 52.