In the Data Grid's toolbar, the "Bottom" button in the grid does not scroll to the end of the results. It only scrolls down to the first 250 records. The arrow keys and scroll bar on the right does not scroll down to the rest of the results that should have been returned.
The session buffer, under View | Preferences | General | Session | Read Buffer Size is by default set to 250. This will limit the number of row first retrieved by the query in the editor. If there are more rows, the rest will need to be fetched before more can be viewed in the grid.
The "Bottom" button will only take you to the bottom of the available data in the grid.
To scroll down and seem more data, use the "Fetch more" button on the grid's tool bar. Fetch more will grab the next set of data, based off the Read Buffer Size.
To fetch all data for just that one query use the "Fetch All" button in the grid's toolbar. This will grab the complete set of results for that query.
Enable the "Fetch All" option under View | Preferences | Code Editor | SQL Scripts. Keep in mind this will return all data from a query at once and may take a long time to display in the grid if there result set is large. This will apply to the editor and all queries run.