GUI: ClassCastException with FontUIResource and java 21

Client version: 1.4.56-V3 (build: 2025-02-09 16:02)
Java version: 21.0.6
Default charset: UTF-8

class javax.swing.plaf.FontUIResource cannot be cast to class javax.swing.Painter (javax.swing.plaf.FontUIResource and javax.swing.Painter are in module java.desktop of loader 'bootstrap')

Stack trace:
java.desktop/javax.swing.plaf.nimbus.NimbusStyle.getBorderPainter(NimbusStyle.java:791)
java.desktop/javax.swing.plaf.nimbus.SynthPainterImpl.paintBorder(SynthPainterImpl.java:117)
java.desktop/javax.swing.plaf.nimbus.SynthPainterImpl.paintPanelBorder(SynthPainterImpl.java:975)
java.desktop/javax.swing.plaf.synth.SynthPanelUI.paintBorder(SynthPanelUI.java:196)
java.desktop/javax.swing.plaf.synth.SynthBorder.paintBorder(SynthBorder.java:63)
java.desktop/javax.swing.JComponent.paintBorder(JComponent.java:1021)
java.desktop/javax.swing.JComponent.paint(JComponent.java:1129)
java.desktop/javax.swing.JComponent.paintChildren(JComponent.java:961)
java.desktop/javax.swing.JComponent.paint(JComponent.java:1137)
java.desktop/javax.swing.JLayeredPane.paint(JLayeredPane.java:586)
and other 26 lines

Start by reproducing the GUI failure with client version 1.4.56-V3 on Java 21. The stack trace enters javax.swing.plaf.nimbus.NimbusStyle.getBorderPainter and SynthPainterImpl.paintBorder; trace the GUI setup that reaches those Swing paths and confirm the ClassCastException is resolved under Java 21.

Written by the indexing model from the issue text.