For invalid X/Y values passed to methods javax.swing.plaf.nimbus.AbstractRegionPainter.decodeX/Y() java.lang.Assertion error is thrown which is unspecified. It is assumed that IllegalArgumentException would fit better for this case.

Please see the following code sample:
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import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;

public class Test {
    public static void main(String[] args) {
        class MyARP extends javax.swing.plaf.nimbus.AbstractRegionPainter {
            MyARP() {
                // workaround for CR#6849331
                paint(new BufferedImage(1, 1, BufferedImage.TYPE_INT_RGB).createGraphics(), new JButton(), 10, 10);
            }
            protected PaintContext getPaintContext() {
                // workaround for CR#6849331
                return new PaintContext(new Insets(1, 2, 3, 4), new Dimension(100, 100), true);
            }
            protected void doPaint(Graphics2D g, JComponent c,
                                   int width, int height,
                                   Object[] extendedCacheKeys) { }
            void call() {
// decodeY(
                decodeX(
                        -1.f
                );
            }
        }
        new MyARP().call();
    }
}
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Execution will lead to:

Exception in thread "main" java.lang.AssertionError: Invalid x
at javax.swing.plaf.nimbus.AbstractRegionPainter.decodeX(AbstractRegionPainter.java:242)
at Test$1MyARP.call(Test.java:21)
at Test.main(Test.java:26)