Waits for a change in the visibility state (as indicated by the arrival of a <VisibilityNotify> event) of window name. This is typically used to wait for a newly-created window to appear on the screen before taking some action. Returns an empty string.
Note that this command is not particularly useful on non-X11 platforms (Windows, OSX/Aqua) because <VisibilityNotify> events are never delivered. This is a platform limitation.