focusUp method
void
focusUp()
Implementation
void focusUp() {
// print(indexWithFocus);
if (indexWithFocus == Index(-1, -1)) return;
if (indexWithFocus.row != 0)
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();
}