I have my tags organised hierarchically, e.g. "TopicA|SubtopicX" etc, and I often want to apply a tag based on the subtopic, but the Add button (under Tags) can only auto-complete the beginning not also the middle of tag names, which means you need to type out the whole of the "prefix" such as "TopicA|.." before you get your desired subtopic.

This is unlike the filter function of the tags window on the left-hand side, which does recognise middles as well.

In my opinion, it would be really useful if the Add Tag button had this feature as well.