Thanks to everyone who responded.  After many hours (many) I finally got it working using the advice I received.  These corner cases really eat my lunch.

Thanks again

Hi James,

You are right: Filtering of dropdown when multiple rows visible is NOT straightforward.

For argument's sake I will name like this:
Main-DW: DW displayed on screen having a DDDW column.
Child-DW: DW displayed as dropdown for the DDDW column.

Child-DW's primary buffer acts as a lookup table: Data displays with display value when data available in primary buffer.

Now, filtering moves all rows failing the filter-expression into the filter buffer. So such data-display rows are unavailable for the Main-DW.

There are several ways to circumvent this issue and still display the "display" value instead of "data" value for all non-current rows in Main-DW.

1. No filtering of Child-DW, instead have rows display or hide in dropdown using other techniques.
   Techniques:
   1. Each of my techniques in this case use extra column in Child-DW:
      Column Name=IsActive, Type=long, Initial=1.
      Custom filtering function: Rows passing filter => IsActive=1,   Rows failing filter => IsActive=0
      
      
   2. OPTION A: Hide rows based on IsActive.
      Detail band has AutoSizeHeight=true and Height=0.
      Visible attribute on all objects in the detail band: if(IsActive=1, 1, 0)
      
      
   3. OPTION B: "Disable" rows based on IsActive.
      Set Font.Strikethrough expression = if(IsActive=1, 0, 1) <= Normal or stricken out
      Set Font.Color expression = if(IsActive=1, 0, ‭8421504‬ ) <= Black or grey(128, 128, 128)
      
      
   4. For both options: ItemChanged must fail any selection where IsActive = 0.
      
      
2. Filter but use other technique to display "display" values for all non-current rows despite no row in Child-DW's primary buffer.
   Techniques:
   1. OPTION C: Two DDDW columns display on top of each other.
      Column having filtered DDDW > Visible = if( GetRow( ) = CurrentRow( ),  1, 0 )
      Column having unfiltered DDDW > Visible = if( GetRow( ) = CurrentRow( ),  0, 1 )
      This has resource consumption impact because you allocate an extra DDDW displaying all rows 
      Little performance impact though because you could do RowsCopy from one DDDW to the other instead of retrieving same data into both DDDW.
      
      
   2. OPTION D: Filter on/off like your link describes in detail.
      This has performance impact because filter is being turned on and off every so often

I have used option B successfully because it works similar to having set of options where some are enabled, others are disabled, but all options appear in place just displaying differently. No surprise to the user like "why is my presumed 'option' gone?" Instead: "Ah! Disabled, so right now I can't choose this specific value.

HTH /Michael

Now that I think of it, there might be another way of doing this:

- use only one field with the dddw

- instead of filtering the data, somehow set the columns in the dddw to invisible. It's visibility expression would be depending on the value(s) you assign in a hidden field of the dddw. 

- if we're lucky, using the "shift to above" functionality would make the rows appear correctly above each other, without leaving empty white space.

It's 02:45 here, so maybe somewhere this weekend, I'll try to make a sample to illustrate what I mean (and see if it's possible).

"Edit: I realize this is exactly what John has answered before" 

One 'ugly' way of doing this, would be by using a second column, below the first one, with the same dddw assigned (and which you do not filter). That one is only visible if the currently selected/active row is NOT the row it's shown on. For the current row it would have to be invisible and instead you'd show the filtered first dddw.

In other words both fields need an expression in their visible attribute to show when current row or not.

This won't be possible/easy with a grid dw though as you cannot position 2 fields on top of each other.

Hi, James -

Some years ago I figured out a technique that accomplishes what you're requesting. If I recall, it involved one or more computed fields, one of which occupies the same space as the data column utilizing the DDDW edit style and expressions on the Visible property on the column and computed field. I believe it also required the "always show drop-down arrow" property was NOT enabled on the data column object for best usability/appearance. It's not ù rocket science, but it's not a trivial solution to your issue, either.

Unfortunately, my notes on the subject are at the office and I'm unable to access them until Monday. If no one else is able to assist you this weekend, I will follow up early next week.

Regards, John

Not sure how I would do that.  It is already a dddw.  How would formatting work?

you would use a format expression