Can't add tags by drag-and-drop to items in the results of a saved search
Unlike items selected from "My Library", items selected from the results of a saved search can't have tags added to them by dragging them onto the tag name in the tag selector.
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What I thought I found was that I couldn't drag items from the middle pane onto a tag in the tag selector, if the middle pane contained the results of a saved search. However, that seems to work fine when I test it now. So either it happened due to a more complicated sequence of events than I thought, or it was just superstition or some other kind of error on my part. Better just ignore it for now.