The current interface to the address book has a folder list in the left panel and address cards in the middle panel. (Plus details view on the right.) The addressbook interface is unique in that, when selecting a folder in the folder list, nothing changes in the view. The list shows the addresses from folders that are checked, i.e., to see what contacts are in a particular folder, it is necessary to uncheck all the other folders. This is cumbersome.
Suggestion: In the "Folder properties" of an address book folder, add the option to specify a custom colour. Then, in the list of contacts, show the contacts in that colour. This makes it easy to recognise which contact is in which folder.
Alternative suggestion: Use the same UI logic as in other software and get rid of the check boxes in the folder list. Then the contacts list should show the contents of the selected folder(s). (If no folder is selected, it should show either nothing or everything.)

Reproducible: Always