With tagOrId, set the keyboard focus for the canvas to the first item given by tagOrId that supports the insertion cursor; the focus is left unchanged if no such item exists. If tagOrId is an empty string, reset the focus so that no item has it. With no argument, return the id of the item that currently has the focus, or an empty string if none does. An item only displays the insertion cursor when both it is the focus item and its canvas has the input focus.