This program produces the bold version of the default font using Font.font(defaultFont.getFamily(), FontWeight.BOLD, defaultFont.getSize()). That is the same font as the one you get with a CSS rule such as -fx-font-weight: bold, so we can expect that people will be seeing it often. On both Windows and Mac the spacing is wrong, as you can see in the attached screenshots. I suspect that the font metrics are being scaled to non-integer widths, but whatever the reason it is obviously not right for the spacing of the same letters to be different depending only on what letters precede them.

I do not think that this is a problem with sub-pixel positioning as in RT-14423.

public class DeleteMe extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override public void start(Stage stage) throws Exception {
        Font defaultFont = Font.getDefault();
        Font defaultBold = Font.font(defaultFont.getFamily(), FontWeight.BOLD, defaultFont.getSize());
        Label background = LabelBuilder.create().text("Background color").font(defaultBold).build();
        Label foreground = LabelBuilder.create().text("Foreground color").font(defaultBold).build();
        VBox vbox = VBoxBuilder.create().children(background, foreground).build();
        StageBuilder.create().scene(new Scene(vbox)).visible(true).applyTo(stage);
    }
}