Instead of saving hidden pixels, that isn't what you do. You will have a collection of all the objects that are on the stage. In each object you will store information of what is to displayed, how big it is, and where it should be displayed. Then you will have a subroutine that you pass the object key or index to and it will look at all that information and decide where to draw the selection rectangle and handles.
To draw everything, you just loop through the collection and read all the information and draw everything accordingly. Then you decided what is selected and then you run your selection box subroutine. Get it? Email me for further help.