ADDITIONAL SYSTEM INFORMATION :
OS:
macOs Big Sur, 11.0.1

java version:
openjdk version "15" 2020-09-15
OpenJDK Runtime Environment Zulu15.27+17-CA (build 15+36)
OpenJDK 64-Bit Server VM Zulu15.27+17-CA (build 15+36, mixed mode, sharing)

openjfx version:
15.0.1

A DESCRIPTION OF THE PROBLEM :
Input any emoji using Chinese input method in JavaFX text area, an exception is thrown:

Exception in thread "JavaFX Application Thread" java.lang.StringIndexOutOfBoundsException: begin 0, end 2, length 1
at java.base/java.lang.String.checkBoundsBeginEnd(String.java:3734)
at java.base/java.lang.String.substring(String.java:1903)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleInputMethodEvent$7(GlassViewEventHandler.java:647)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleInputMethodEvent$8(GlassViewEventHandler.java:640)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:412)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleInputMethodEvent(GlassViewEventHandler.java:639)
at javafx.graphics/com.sun.glass.ui.View.handleInputMethodEvent(View.java:648)
at javafx.graphics/com.sun.glass.ui.View.notifyInputMethod(View.java:977)
at javafx.graphics/com.sun.glass.ui.mac.MacView.notifyInputMethodMac(MacView.java:161)

Seems that the notify str only contains the first byte of UTF-8 encoded emoji bytes.

Copy & Paste a emoji to text field works fine.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a simple JavaFX app with a TextArea， and Run:

public class Test extends Application {

    @Override
    public void start(Stage stage) {
        stage.setTitle("Test");
        var text = new TextArea();
        var scene = new Scene(text, 600, 400);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

2. Input text using ‘简体拼音’ or any other Chinese input method, for example, keys 'keai' and then select the emoji in the candidate words.