insertBelow method
Inserts an entry below above in the overlay stack.
Implementation
void insertBelow(OverlayEntry entry, {required OverlayEntry above}) {
final index = _entries.indexOf(above);
setState(() {
entry._overlayState = this;
if (index >= 0) {
_entries.insert(index, entry);
} else {
_entries.insert(0, entry);
}
});
}