From The Document Foundation Wiki
How do I select an object in the background
Use the key combination Ctrl+Click.
You can also use the tool Select in the Drawing toolbar (▸ ▸ ) (first tool on the left of this toolbar) to surround the object to select:
- click and keep the mouse button pressed;
- move the mouse to surround the object (the selection is displayed by a dashed line around); and
- release the mouse button: the object is selected.
The Navigator (▸ menu or F5) also allows you to select most of the objects. However, the drawn objects e.g. rectangles, text area, lines, etc.) are only displayed if they are explicitly named (right-click on the object, then Name).