After editing tags on a bunch of resources, filter the Library, and there’s a focus rectangle left behind on the cover icon for the first item in the (filtered) Library listing.
To reproduce:
Open the Library in Browse mode and turn on the Info panel
Select two or more resources
Click add tag, and type in something, press Enter.
Type anything in the Find box to filter the Library.
You're left with this display artifact:
It stays even after the "Updating library catalog..." message goes away.
