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How would you go about detecting when a user is about to leave your site? Other than responding to a click on an external link? The two ways I typically leave a site are a) typing a URL into the address bar and b) closing a tab.
Watch for the mouse along the upper 10px of the window [0] or use of the [Ctrl] key (for Ctrl+L or Ctrl+W). Or as other said, for a threshold of upper/upper-left mouse movement. You'll get a few false flags from people trying to Ctrl+C but so many people use the right-click context menu to copy/paste...you can probably ignore Ctrl+_ entirely and just focus on that window near the top. Most users will pass by that are…
If somebody is interested in subscribing, then a subscribe box at the bottom or side of the content is fine.