src/java.desktop/share/classes/javax/swing/plaf/metal/MetalToggleButtonUI.java

*** 210,220 ****
                  g.setColor(c.getBackground());
              } else {
                  g.setColor(getDisabledTextColor());
              }
          }
!         SwingUtilities2.drawStringUnderlineCharAt(c, g, text, mnemIndex,
                  textRect.x, textRect.y + fm.getAscent());
      }
  
      protected void paintFocus(Graphics g, AbstractButton b,
                                Rectangle viewRect, Rectangle textRect, Rectangle iconRect){
--- 210,220 ----
                  g.setColor(c.getBackground());
              } else {
                  g.setColor(getDisabledTextColor());
              }
          }
!         getTextUIDrawing().drawStringUnderlineCharAt(c, g, text, mnemIndex,
                  textRect.x, textRect.y + fm.getAscent());
      }
  
      protected void paintFocus(Graphics g, AbstractButton b,
                                Rectangle viewRect, Rectangle textRect, Rectangle iconRect){