Not really a bug but rather a small cosmetic annoyance. The contents of expanded folders in the sidebar are indented too much to the right. It looks weird and takes too much horizontal space. Compare Pedia with other applications. For example, the Finder in List View. Notice Pedia's indentation reserves space for the collapse/expand arrow of nested folders, whereas Finder doesn't.
It's easier to see in pictures:
Thanks for your attention.