Those folders are real folders on your hard drive.
By default, your Shotwell folder is just ~/Pictures.
Open Nautilus (the file manager), and navigate to it. You will see the parent folder (e.g. 2012, then go to the empty folder, e.g. December) and then you can delete it.
Click for bigger imag...
It looks like you're referring to folders underneath the Events section; I am aware of the correspondence between the Events descendants and the filesystem structure. I'm not talking about that; there is a separate section, a sibling of Events, called "Folders", and that's what I'm asking about.
Tim, thanks, I do realize that they correspond to real folders, but the other folders in the Folders section of the sidebar disappeared once I removed their images from my library. There is a descendant of Folders for me that contains zero images, but it's still present under Folders. I'd like to know why it's showing up and how I can delete it from Shotwell (without deleting it on the filesystem).
Tim, I tried it, and it didn't work. When I dragged it to the folder in Shotwell, it imported it as normal into its own Event folder, but it didn't show up in the Folders folder...
I backed up my photo.db file, and then, using sqlite3, deleted the three entries in PhotoTable whose filename value matched that folder. I restarted Shotwell and that folder was gone. If I can provide you those three records from my photo.db file, will that give you enough info to know why this was happening? I'm considering filing a Shotwell bug for this, but I need to confirm with someone that it's actually a bug...
Discussion between BlueBomber and Tim
Discussion between BlueBomber and Tim