The middle mouse button doesn’t trigger a click event, but rather auxclick (like right click also doesn’t, but rather contextmenu and then auxclick if that’s preventDefaulted).

But there are definitely situations where it’s judicious to check event.button, and I missed altKey, too. Here’s the full function Fastmail uses:

  const isClickModified = function (event) {
      return (
          !!event.button ||
          event.altKey ||
          event.ctrlKey ||
          event.metaKey ||
          event.shiftKey
      );
  };

Ah indeed, I probably confused it with other mouse events that do not have a button-specific event like `auxclick` and `contextmenu`. Mouse down/up only use the button to differentiate the buttons, which can also explain why that function includes such check (even if unnecessary on the `click` event)

On a side note, I can't believe we still don't have a `visit` or `activate` event that works regardless of hardware, without having to exclude modifier keys.