It is a gopls snippet when completing struct literal field names. You get the comma if the field name is on a different line than the open curly.
This could probably save me multiple keypresses per week.
Granted, but we need to weigh that against how many keystrokes it takes to implement the feature.
Jokes aside, I think this could work, but gopls would have to leave the cursor before the comma. If the user intends to add more to the expression it would be annoying for the cursor to jump to the far side of the comma after accepting the first completion.