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While the Table Pre-Write and Post -Write Events can handle when a user exits a line, I need to set Line UDF values as soon as a user selects any field in a line grid.
If you're on v2015 there is a new UI script trigger that can fire when a tab is clicked, which is probably as close as you can get. In Customizer, right-click PLINES (for your SO panel), and choose the UI Script Events option.
It sounds like the Post Load happens once when the grid is accessed, like the Table-Script Initialization event. Or does it happen with each selection of a line?