The underline of the top row is hidden by the background of the bottom row

[PavelTurk]

This is test code:

public class RichTextFxTest extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        var textArea1 = new InlineCssTextArea();
        var css = "-rtfx-underline-color: #f80000ff;-rtfx-underline-width: 1px;"
                + "-rtfx-underline-offset: 2px;-rtfx-underline-double-gap: 1px;";
        textArea1.append("AAAAAAAAAAAAAAAAAAAAAAAAA\n", css);
        css = "-rtfx-background-color: #FFFF00ff;";
        textArea1.append("BBBBBBBBBBB\n", css);

        var box = new VBox(new Label("TextArea1"), textArea1);
        var scene = new Scene(box, 300, 300);
        stage.setScene(scene);
        stage.show();
    }
}

And this is the result:

I am not sure, but maybe the lines of the top row must be above the background of the bottom row?

[Jugen]

Yeah, I'm not sure how to fix that.
BTW it behaves correctly for multi-line paragraphs, but in your example each line is its own paragraph and then doesn't render correctly because each line is independent of the next.