Default virtual rendering of grid is limited to its viewport i.e., only the cells viewable in the viewport at any given time are rendered.
If we set height: 'flex' for large number of records as in this example (~10,000 records), then it causes the grid to have a quite large viewport which makes the default virtual rendering of the grid of little use.
It may even crash the browser.
virtualWinv8.1.0 introduces a new option virtualWin which renders the cell based on the intersection of browser window viewport as well as the grid viewport.
This helps to make the rendering of cells quite fast as in this example.