This used to be a synchronous check done through a cached hasTags API, but we can't do synchronous querying here. Thus, bug 1477930 broke the open glyph on the tags root. A possibility may be to pass hasChildren as an argument when we create a container, and then keep it up-to-date using notifications. This may even be cheaper for other cases, and it's usually easy to query for children when we query for the other container information.
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