Unfortunate issue with dependent DDDW on InfoMkaer custom tab

Hi mes amis 

I have an *interesting* Infomaker challenge (as in the alleged Chinese curse).

I'm making a constituent custom tab, in InfoBreaker. (Actually, I'm doing a new version of an existing custom tab, and I don't want to start again from scratch.)

On this screen, I have a dropdown which wants to allow you to select an order_no, of a particular kind of order, belonging to the constituent. The screen then lets you add some fairly elaborate custom order data to a local table. 

So it's a dependent DDDW. I haven't made one of those before. I've followed the slightly obscure and not entirely accurate instructions in the custom tabs doc, involving placing two carefully configured text controls precisely in the same space, one in front of the other.

1. I save my infomaker datawindow(s), and pop them into my test environment.
2. I open a new Tess session, to be sure.
3. I open a constituent, go to the custom tab - Shazam! It works! I can select orders from the dropdown, add, edit, delete records, no worries.
4. I close that constuent, and open another one to do some more testing.
5. ...It doesn't work anymore. The dropdown drops down ok, but there's nothing in it. No orders to pick from. Zilch. Nada.
6. I go back to my original constituent. Doesn't work there anymore either. Or anywhere else.
7. Go back to Step 1 and around again.

The only thing I can think of is that doing the selections in the first constituent record has somehow disturbed the delicate equilibrium of the datawindow, so that the two controls are no longer aligned correctly. Is that even possible?

Or perhaps there's a much simpler explanation, and a fix which one of you lot will swiftly respond with for me, and I can go on with my life????

All suggestions gratefully received. Particularly ones that work. (And I know there's a quite sensible suggestion that goes "Throw it away and rebuild the screen as a webpage and never work with Infomaker again"... which is beginning to appeal, but I'm not quite ready to do that yet.