When running test javax/swing/text/BoxView/6494356/bug6494356.java with VM options -XX:+UnlockDiagnosticVMOptions -XX:DiagnoseSyncOnValueBasedClasses=1, the VM detects an attempt to synchronize on an instance of a java.lang.Boolean:

# Internal Error (synchronizer.cpp:485), pid=51719, tid=51740
# fatal error: Synchronizing on object 0x00000000e7717b60 of klass java.lang.Boolean at bug6494356.main(bug6494356.java:60)
#
# JRE version: Java(TM) SE Runtime Environment (23.0) (build 23-internal-2024-02-15-2215338.frederic.parain.jdk)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (23-internal-2024-02-15-2215338.frederic.parain.jdk, mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0xe787eb] ObjectSynchronizer::handle_sync_on_value_based_class(Handle, JavaThread*)+0x4db
#

java.lang.Boolean is one of the classes to be migrated to be a value class in JEP 401 Preview, which would cause this test to fail even without the DiagnoseSyncOnValueBasedClasses diagnostic option.