Avoid binding multiple mousedown handlers to body.
Avoid rebinding deprecated focus/blur callbacks more than once.
Delay cleanup of delegated event handlers on deactivation till last

If you trigger 'hide' after you've cleaned up the event handler, it's not
going to work is it?
Don't glue link popover to math tooltips

Specifically, the X that removes math is an <a>, but it isn't a link. This is
a generic fix that won't place popovers on anything that is marked as ephemera.