I updated the clone example with a couple more experiments. One was my current thoughts for a workaround (putting the Text Box directly into the row, but then putting a Flow Panel for the other stuff beneath it. This takes up more screen height and I don’t like it, but it works).
The other was to eliminate the Data Grid from the equation and just try a plain Text Box versus a Text Box in a Flow Panel on the form itself. Those behaved the same as inside the Data Grid.
I’m not certain what is responsible for expanding the Text Box when it’s placed directly into the form’s content panel. CSS? Editor code? Javascript of the Text Box? The content panel?