The last line of a buffer is not correctly highlighted

[registerGen]

Describe the bug

The last line of a buffer is not correctly highlighted.

Steps to reproduce

Paste the following code, and edit the main() function.

#include <iostream>
using namespace std;

int main() {
  int a, b;
  cin >> a >> b;
  cout << a + b << endl;
  return 0;
}

Expected behavior

The last line is highlighted by red.

Screenshots

After editing: