Currently theStreamedJsonResponse only supports streaming nested Generators within an array data structure.
However if a response is a list of items (for example database entities) on the root level, this isn't usable.
I think both usecases can be supported with the change in this PR.

The root level generator doesn't account for additional nested generators yet. I could add that by doingis_array($item) and the call the recursive placeholder logic.

Link to first PR that introduced StreamedJsonResponse:#47709

Also something I noticed is I only got intermediate output, when adding aflush() call after each item has been echo'd (with asleep(1) after each item to see it output the parts individually). Edit: I see the class' PhpDoc describes this and it's probably expected to be done in userland implementations.