it's the window.onblur handler. If you select "inspect element" the browser opens a new window for the developer tools and the previous window loses the focus,

it's the window.onblur handler. If you select "inspect element" the browser opens a new window for the developer tools and the previous window loses the focus, triggering the handler. It also happens if you load the page then open a new tab. The previous tab will lose focus and close itself.