Virtual Content: Reverse navigation

[LJWatson]

In the Virtual Content explainer, the last two steps of the scenario walkthrough say:

13. Eventually, the backing server sends a content payload which also contains a signal that the end of the document has been reached. Script code removes
    the aria-virtualcontent attribute from the main element to indicate there is no more virtualized content.

14. The next time the user attempts to navigate to the next heading, the AT searches for a virtual content container, finds none, and announces there are
    no further headings in the document.>

In this scenario, what would happen if/when the screen reader user decides to reverse direction and move backwards through the headings?

[atanassov]

The behavior of the feature is intended to be symmetric in both the block and inline directions. For this very reason we're planning for a value set of the attribute that would allow authors to hint if there is content forward, backward in the block or inline direction. The canonical example here being a large excel spreadsheet with many columns overflowing in the inline direction and many rows in the block direction.