We should have some tests for the event sequence when focusing a different-site iframe by clicking inside it.

At present, my hypothesis is that the Web doesn't require the JS-observable focus event to fire from the same event loop task as the JS-observable user input event that caused the focusing, e.g. mousedown.

@mfreed7, do you know if Chrome guarantees anything about same or different task here, especially in the case where an out-of-process iframe doesn't have focus and gets focused by click? Any lore on whether there are Web compat constraints on this? (At present, Firefox fires the events from the same task in the in-process case and from different tasks in the out-of-process case.)