public interface CustomTabElements
<input> element is written into  some Canvas as part of its
 contents, a developer would likely want this element to be included in the tab sequence for the
 page in its intuitive spot (between the canvas it is embedded in and any subsequent canvases on
 the page). To achieve this a developer needs to take several steps:
<input> element
 written into a Canvas, the developer      would choose an arbitrary identifying string for the
 input element, and call the      TabIndexManager.addTarget() API, passing in the      ID of the canvas in which it is embedded      as the
 parentID parameter.TabIndexManager.getTabIndex()
 to retrieve a a numeric tabIndex to apply to the     element. This can be used when generating
 the HTML to write out for the element.moving the canvas containing the item to a new
 parent     would cause this value to change. The tabIndexUpdated callback
 parameter     of the TabIndexManager.addTarget() method is a notification which will be     called when this
 occurs, and may be used to update the element's tab index if     it has already been written
 into the DOM when the value changes.click mask is up, using the Tab key to
 navigate between editors embedded in a ListGrid during grid editing, and for any focusable UI embedded in a canvas where Canvas.alwaysManageFocusNavigation has been explicitly set to true.shiftFocusCallback parameter should be passed to 
 TabIndexManager.addTarget() when
 registering an ID for the custom element.      This is a callback developers should implement
 to     put native focus into the target (or return false if this is not currently possible    
 for some reason). It will be invoked automatically by the system when focus is being    
 shifted programmaticallyTabIndexManager.useExplicitFocusNavigation(), passing in the ID that was     registered via
 TabIndexManager.addTarget()), and
 if that returns true     explicitly calling TabIndexManager.shiftFocus(), and
 preventing default     native behavior by invoking     preventDefault()     on the native event object.