When the instance health page is accessed, it schedules some checks in the background, and pops a loading spinner onto the page. If browsing to another tab, when the loading completes it won't remove the spinner.
Load instance health.
Browse to another tab before it finishes loading.
The spinner has vanished.
It doesn't go away, as we use fadeOut and https://bugs.jquery.com/ticket/9310 is closed as won't fix.