Number of rendered items in BitBasicList component in virtualize mode

[maryamhdr]

We expect the number of rendered items in BiBasicList be equal to number of items shown in the list + 2 * OverScanCount, but the actual rendered item count is greater than this expecting value.

[msynk]

Although the general idea of this issue is not wrong, the expected number of rendered items is not always equal to:

N + 2*OverScanCount

the marginal cases are at the start and end of the list in which the buffer before or after the actual list does not exist.