[TJSomething]

Ah. They're using the pointerdown event, which was added in Firefox 59 (https://developer.mozilla.org/en-US/docs/Web/API/GlobalEvent...).

[raphlinus]

Right, this is also why it doesn't work in Safari. That's fixed now, but imperfectly (I don't know how to do the equivalent of setPointerCapture in ancient browsers, and also it only adds a mouse, not a touch handler, so still doesn't work in mobile Safari). Thanks!