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Folder items in lists may be opened or closed by double-clicking on or touching an expand/collapse visual indicator displayed right next to an item's icon. The indicator clearly shows, if its item is expanded to show the next level of items, or collapsed, hiding all its sub items. If a folder item is expanded, only the next level of its sub items is shownor 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, a visual indicator asks the user for patience. Clicking on or touching an expanded folder item collapses it again and hides all of its sub items.
Using 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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