- Wait for a specific event, the observer callback being called, when
   possible. This avoids a possible issue where
   `ResizeObserver`/`MutationObserver` callbacks would not be delivered
   within the fixed time
 - Fix a typo in the name of the test file

Original issue report: https://hypothes-is.slack.com/archives/C1M8NH76X/p1574692275043700