focusDown method
void
focusDown()
Implementation
void focusDown() {
if (indexWithFocus == Index(-1, -1)) return;
if (indexWithFocus.row < rowOffsets.length - 2)
indexWithFocus = Index(indexWithFocus.row + 1, indexWithFocus.column);
cellSelection.clear();
cellSelection.add(indexWithFocus);
rowSelection.clear();
rowSelection.add(indexWithFocus.row);
columnSelection.clear();
columnSelection.add(indexWithFocus.column);
selectionChanged?.call();
}