C++ :: Program Breaks If Copy Stuff With Multiple Lines Into Console - Clearing Input Buffer

Using cin.sync() works great so far, but my program still breaks if you copy something with multiple lines into the console.

string test = "";
while(true) {
cin.sync();
getline(cin, test );
cout << endl << "test: " << test << endl;
}

However, if you were to copy this:
1
2
3

and paste it into the program, the output would be1
2
3

test: 1
test: 2

And if you press enter one more time:1
2
3

test: 1
test: 2
test: 3

The 3 finally pops out.