@Retool.
The answer in part anyway is in this post.
- In the Inspector panel for the Table (New), click on the equalizer icon next to Interaction
- Find the "On Action Click" dropdown and click on it
- Do Not Select the Row is the default, which is why SelectedRow is undefined when the user clicks on the Action button (if no row is already selected)
- Choose either to replace any current row selection with the row over which the Action button was clicked or to add the row to the existing collection of selected rows.