BeginMultiSelect() item count copied for request handlers; -1 if unknown.
Last known signed 64-bit selection user data for the navigation-focused item.
Last known selection state of the navigation-focused submitted item.
Range anchor's signed 64-bit user data; ensure its item is not clipped.
Set before EndMultiSelect() to reset the range anchor, e.g. after deletion.
Multi-selection range anchor and navigation state, separate from requests.