This is a bit peculiar. When we show the bookmarks toolbar, we create its children. When it then
gets hidden, they do not get removed. When we then re-enable the bookmarks toolbar, we call
`rebuild` on the next tick (because of the `await` in `init` in PlacesToolbarHelper), which
synchronously removes all the children of the toolbar, and then rebuilds the places node. It
does that async because it avoids doing synchronous layout flushes.
This breaks in the test - and the closer you put the "show the bookmarks toolbar" code to
the code that tries to open the context menu, the more reliably it breaks, because we try
to show the context menu right after we synchronously remove everything, and before we've
(asynchronously) put content back.
To avoid the raciness here, we wait for the custom event in the places toolbar to indicate
we're done building it.
Differential Revision: https://phabricator.services.mozilla.com/D116118
38c437fc9a