Folders and organisation
The library is a folder tree. Folders nest as deeply as you like, and a test case belongs to exactly one — which is also how it belongs to a project, since the folder carries that.
Working with the tree
The left pane of Test Cases is the tree; the right is whatever the selected folder holds. Selecting All test cases lifts the folder filter and shows the project's whole library.
Folders can be created, renamed, moved and deleted from the tree's context menu. Deleting one cascades: its subfolders, their test cases and those cases' steps all go with it, and there is no undo. Only owners and admins can delete.
Colours
A folder can be given a colour, which is carried into the lists and pickers that mention it. This is the cheapest way to make a large library scannable — one colour per area, and the eye does the filtering.
Following a folder
Click Watch folder to be emailed when test cases are added to it.
A subscription covers the whole subtree: following Checkout also catches
Checkout / Payments. Deleting a folder cleans up the subscriptions pointing at
anything inside it, in both directions — somebody following
Checkout / Payments loses what they followed when Checkout goes.
Filtering rather than foldering
Folders are one axis. Before building a deep tree, consider whether tags do the job better: a case can carry many tags but only one folder, and a bug hunt can pull in every case carrying a given tag. Deep trees are hard to move through; a flat tree with good tags usually is not.
Every list screen also carries the same toolbar: a search box, a filter menu whose selections go into the URL, and a reset. A filtered list can be linked to and shared, because the filter is in the address.