In this situation:

```
<button popovertarget=foo>Activate
  <div popover id=foo>Clicking me shouldn't close me</div>
</button>
```

clicking the button properly activates the popover, however, clicking on the popover itself after that should **not** close the popover. It currently does because the popover click bubbles to the `<button>` and activates the invoker, which toggles the popover closed.

This patch fixes that case.