Send to Back: Design Edit Menu

Sending the object(s) to the back allows you to see and select the other objects that were behind it. 

Select the desired object(s).

Click Edit > Send to Back to send the currently selected object(s) to the back of the other objects, including Border lines, Guide lines. 

Select the desired object(s).

Click Edit > Send to Back.

The object will be sent to the back.

This may be faster than hitting PgUp/PgDown to cycle through the openings in order to select an object hidden by another opening.