null where there is nothing to offer: the destination does not extend the template (a
release on the handle's own corner, or a shrink), in which case the drag is best treated as a
left-drag — a shrink clears whichever way the button was held.
Every question the menu has is answered here so the menu itself renders labels and calls back:
which types apply is ICellRange.getDefaultAutoFillCriteria asked in its drag form, and a
chosen entry runs through the same autoFill a left-drag runs, with the destination selected the
same way afterwards.
The fill menu's context for a right-drag of the fill handle from
templatetodestination— Excel's menu of Copy Cells / Fill Series / Fill Formatting Only / … / Series…