JS mouse events that work even when mouse is moved outside the window

It's not guaranteed that mousedown will be followed by mouseup on any element in the document!

If you ever expect mousedown to happen on any DOM node, you're creating a "sticky mouse button" bug.

document.body.onmouseup = // NO, this is a bug!

Instead you should only listen to mouseup on window:

window.onmouseup = function(e) {…}

or better:

window.addEventListener('mouseup', function(e) {…}, false);

The same applies to mousemove event. It will fire even when mouse pointer leaves the window and your scripts tracking the mouse won't go blind and glitchy when mouse pointer moves outside document's viewport.

You can see it in action in Slip.js.