About the widget tree sidebar

The widget tree displays a tree hierarchy of the widgets (panes, labels, graphic elements, and so on) that are in the current Px view.

Figure 1. Widget tree sidebar

It is often easier to use the Widget Tree to select objects when you have a lot of objects on a view–especially when there are several layers of objects. When you select an object in the tree view it is selected in the Px view as well and displays the selection borders and handles.