investigate window.requestIdleCallback() for possible timing leaks
The window.requestIdleCallback() API is available as of Firefox 55. We should determine whether it may be used to learn too much about the performance of the user's computer/device, or if there are other timing leaks we want to avoid. See:
https://bugzilla.mozilla.org/show_bug.cgi?id=1314959
https://developer.mozilla.org/en-US/docs/Web/API/Background_Tasks_API
If necessary, we can disable this feature by setting dom.requestIdleCallback.enabled to false.
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