MWE RTFX style overlay

App.java

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

import org.fxmisc.richtext.InlineCssTextArea;
import org.fxmisc.richtext.model.StyleSpans;
import org.fxmisc.richtext.model.StyleSpansBuilder;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.layout.AnchorPane;
import javafx.stage.Stage;


public class App extends Application {

    private static final String BOXED_STYLE
            = "-rtfx-border-stroke-width: .75pt;"
            + "-rtfx-border-stroke-type: outside;"
            + "-rtfx-border-stroke-color: darkgoldenrod;"
            + "-rtfx-background-color: antiquewhite;";
    private static final String OTHER_STYLE
            = "-fx-font-weight: bolder;"
            + "-fx-fill: blue;";


    @Override
    public void start(Stage primaryStage) {

        CustomCodeArea codeArea = new CustomCodeArea();
        codeArea.replaceText("Lorem ipsum dolor sit amet consectetuer adipiscing elit");

        TextSpanCoordinates firstFiveWords = new TextSpanCoordinates(0, 0, 0, "Lorem ipsum dolor sit amet".length());
        TextSpanCoordinates secondAndThirdWords = new TextSpanCoordinates(0, 0, 6, 16);

        codeArea.setStyle("-fx-font-size: 12pt;");
        codeArea.styleCss(firstFiveWords, BOXED_STYLE);
        codeArea.styleCss(secondAndThirdWords, OTHER_STYLE);
        codeArea.paintCss();

        AnchorPane pane = new AnchorPane();
        pane.setPrefWidth(400);
        pane.setPrefHeight(50);
        pane.getChildren().add(codeArea);
        codeArea.setPrefWidth(400);
        codeArea.setPrefHeight(50);

        primaryStage.setScene(new Scene(pane));
        primaryStage.show();
    }


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


    static class TextSpanCoordinates {
        final int beginLine;
        final int endLine;
        final int endColumn;
        final int beginColumn;


        TextSpanCoordinates(int beginLine, int endLine, int beginColumn, int endColumn) {
            this.beginLine = beginLine;
            this.endLine = endLine;
            this.beginColumn = beginColumn;
            this.endColumn = endColumn;
        }
    }

    static class CustomCodeArea extends InlineCssTextArea {


        private final Stack<StyleSpans<String>> layers = new Stack<>();


        /** Registers a span to be styled. */
        void styleCss(TextSpanCoordinates node, String css) {

            int offset = getAbsolutePosition(node.beginLine, node.beginColumn);
            int spanLength = getAbsolutePosition(node.endLine, node.endColumn) - offset;

            StyleSpansBuilder<String> builder = new StyleSpansBuilder<>();

            builder.add("", offset);
            builder.add(css, spanLength);
            builder.add("", getLength() - (offset + spanLength));

            layers.push(builder.create());
        }


        /**
         * Forcefully applies the possibly updated css classes.
         */
        void paintCss() {
            this.setStyleSpans(0, overlayAll());
        }


        private StyleSpans<String> overlayAll() {

            List<StyleSpans<String>> allSpans = new ArrayList<>(layers);

            if (allSpans.isEmpty()) {
                return new StyleSpansBuilder<String>()
                        .add("", getLength())
                        .create();
            }

            final StyleSpans<String> base = allSpans.get(0);

            return allSpans.stream()
                           .filter(spans -> spans != base)
                           .filter(spans -> spans.length() <= getLength())
                           .reduce(allSpans.get(0),
                                   (accumulator, elt) -> accumulator.overlay(elt, (style1, style2) -> style1 + ";" + style2)
                           );
        }
    }
}