Popovers are broken when shown in a `contextmenu` event listener

### What is the issue with the HTML Standard?

https://codepen.io/jpzwarte/pen/OPLQypz?editors=1010

If you want to use a popover for a context menu, you cannot use the `contextmenu` event to display the popover. The problem is that the popover is immediately light dismissed.

(not sure if this belongs in whatwg/html or whatwg/dom)