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Folder items in lists may be opened or closed by double-clicking or double-tapping its icon or thumbnail image while holding down a modifier key (Command key on Mac, Alt on Windows and Linux). The content of the list view is then replaced by the list of items contained in the folder item. If data about items has to be loaded first, the list view is replaced by a visual indicator asks asking the user for patience. To return to the parent item of a currently shown sub list of items, the list view provides an "up" button - clicking or touching it returns you to the list view containing the parent folder item.
When using a keyboard, first focus the folder item you intend to open, then press the ENTER key together with a modifier key (Command on Mac, Alt on Windows and Linux). To return to a parent list from a sub list of items, press the UP arrow together with the same modifier key (Command on Mac, Alt on Windows and Linux) with the keyboard focus on any item in the list.
A list is not a very convenient way to quickly navigate a hierarchical set of items, but has the advantage of visualising the type of a file or even showing thumbnail previews of an item's contentUsing the arrow keys presents a very efficient way to navigate a tree. It will quickly become one major way to work with trees for any experienced user. Care must be taken therefore to ensure that this will be a smooth, reliable, complete and non-irritating experience.
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