activate method
Move an existing surface to the top without changing others' relative order.
No-op if id is missing or already top. Resets pending on the previous top.
Implementation
bool activate(String id) {
final i = _indexOf(id);
if (i < 0) return false;
if (i == _stack.length - 1) return true;
final prevTop = top;
prevTop?.reset();
final surface = _stack.removeAt(i);
_stack.add(surface);
_notifyStack();
onPendingChanged?.call();
return true;
}