Hide hover card when infonav dropdown is hidden

[matthew-white]

Right now, a hover card is hidden (1) when the user mouses away from the anchor element and (2) when the anchor element is removed from the DOM. However, once we add infonav buttons, I think we will also need to hide the hover card when the anchor element is hidden. I'm thinking of a sequence like the following:

• Click the "Updates X Entity Lists" infonav button. That should show a dropdown of entity lists.
• Hover over the first entity list in the dropdown. That should show a hover card.
• Press the Escape key. That will hide the dropdown. It should also hide the hover card. However, as hover cards are implemented today, it won't hide the hover card.

Maybe we could use an intersection observer for this? Or maybe we don't need to do something generic like that, and we could just have the infonav button be responsible for hiding the hover card as needed.