Testsuite name: Swing manual
Test name(s): Swing_JMenu/Manual/MenuLAFDemo
Product(s) tested:JDK 8u221b06 (64bit)/tonga-4.0.2-rc2
OS/architecture:oel7.6-uek-x64
option: -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel -Djdk.gtk.version=3

Actual behavior:
1.step 1:There is no mnemonic in the menu(MultiLevelMenu). Refer to MenuLAFDemo_with-option.png.
2.No such issue without option.Refer to MenuLAFDemo_no-option.png
3.No such issue on oel6.4-uek-x86/JDK8u221b04(32bit) with the same option