ThreadGroup.stop is terminally degraded in the main line and degraded to throw UOE in the loom repo. This impacts AppContext.dispose which seems to use ThreadGroup::stop to "stop" all remaining threads in the group. This impacts a number of tests with exceptions like the following:

java.lang.UnsupportedOperationException
at java.base/java.lang.ThreadGroup.stop(ThreadGroup.java:533)
at java.desktop/sun.awt.AppContext.lambda$dispose$0(AppContext.java:504)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:312)
at java.desktop/sun.awt.AppContext.dispose(AppContext.java:503)
at bug6190373.main(bug6190373.java:66)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:97)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
at java.base/java.lang.Thread.run(Thread.java:1502)

Tests that fail due to this are:

java/awt/AppContext/ApplicationThreadsStop/ApplicationThreadsStop.java
sun/awt/AppContext/8012933/Test8012933.java
javax/swing/system/6799345/TestShutdown.java
javax/swing/plaf/metal/MetalUtils/bug6190373.java