FULL PRODUCT VERSION :
java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+152)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+152, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.14393]

A DESCRIPTION OF THE PROBLEM :
 Supplementary-plane characters in text fields are sometimes treated as two characters - it must be caused by their internal representation as surrogate pairs.

REGRESSION.  Last worked in version 8u112

ADDITIONAL REGRESSION INFORMATION: 
java version "1.8.0_112"
Java(TM) SE Runtime Environment (build 1.8.0_112-b15)
Java HotSpot(TM) 64-Bit Server VM (build 25.112-b15, mixed mode)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run sample program given below. In opened text field, press Left first, then Backspace.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
First character is deleted (completely), second remains.
ACTUAL -
First character is replaced with some undisplayable character, probably because only a part of surrogate pair was deleted.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.*;

public class JTextFieldTest {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame();
            JTextField field = new JTextField(20);
            field.setText(new String(new int[]{0x1d400, 0x61}, 0, 2));
            frame.add(field);
            frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}

---------- END SOURCE ----------