Clicking the Z Order button brings up the Z Order dialog box, which shows a list of all the Controls and Background Elements on this page, in the order they are being drawn.  (I.e., the current Z Order.)

The item at the top of the list is drawn first, and is therefore behind all the other items in the list.  Unfortunately, our Alpha Box (AL1 id="al1") is at the bottom of this list, indicating that it is drawn last, and is therefore being drawn on top of all the other controls.