Hide tooltip when its anchor element is moved off-screen

[matthew-white]

See getodk/central-frontend#811 (comment). If a tooltip is shown for an element, then the element is moved off-screen, the tooltip should be hidden. Maybe we could set up an intersection observer on the element?

I feel like I've encountered a separate bug related to tooltips and scrolling the page, but I'm not able to rediscover it at the moment. It's possible that such a bug could be fixed at the same time as this one. I'll update this description if I encounter other tooltip issues that seem related to this one.