When a user drags the mouse across one or more textual characters,
they become highlighted and remain highlighted until the user clicks
or highlights elsewhere. While a selection of text is highlighted, the
user may perform four editing commands. "Copy" puts a copy
of the selected text in the system's 'Clipboard'. "Delete"
removes the selected text and clears the highlight. "Cut" performs
a 'copy' and then a 'delete'. Once a selection has been copied or cut
and placed on the clipboard (so the clipboard is not empty), a "Paste"
will insert that clipboard text wherever the cursor is currently situated,
without removing the item from the clipboard. If the cursor is not active,
or a non-textual item is selected, "Paste" will have no effect.
Similarly, if no text is selected, or a non-textual item is selected,
"Cut", "Copy", and "Delete" have no effect.
Every action the user performs is logged as a "snapshot" of the currently
opened project. Only one snapshot is maintained at a time. When the user
invokes the "Undo" command, the project is reverted to the snapshot,
effectively undoing the last user action. The "Redo" command will
reverse the process, redoing the action. The undo command itself is logged,
thus two invokations of 'undo' in a row yield the same result as one 'undo'
followed by a 'redo'.
2.9.1. Cut, Copy, Paste, and Delete