This is to better help differenciate between the folder vs. message trees. Ideally, the label for the threadTree element would say "messages" when it displays messages in a flat view, and "Threads" when displaying messages in a threaded fashion. For the folder pane, there is actually a label which I hadn't noticed before which can be associated with the folderTree element. For the threadTree element, aria-label should be used to associate a literal localizable string, which should reflect the current display. Should future UI revisions add more trees, labels should be added to those as well so screen reader users get good feedback on what each tree does.
This should still be done I think.
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