The row type.
Every line to draw, in order.
The ordinary rows among displayRows, in order: the page itself when not
grouped, the rows loaded inside expanded groups when grouped.
Fetch the next block of an open group; also retries after an error.
Patch rows already loaded inside groups, matched by id. No request is made; ids that are not loaded are ignored.
Close every group and drop every loaded block, cancelling fetches in flight. Called for you whenever the query changes.
Open a closed group (fetching its first block if needed) or close it.
What useRowGrouping returns.