<Swing Dev> [9] Review Request for 8145787: [TEST_BUG][PIT] javax/swing/undo/UndoManager/AbstractDocumentUndoConcurrentTest.java fails with CannotUndoException

[Semyon Sadetsky]

Hello,

Please review fix for JDK9:

bug: https://bugs.openjdk.java.net/browse/JDK-8145787
webrev: http://cr.openjdk.java.net/~ssadetsky/8145787/webrev.00/

This is a multithreading stress test in which the undo queue depth may 
reach zero sometimes occasionally. The corresponding checks were added.

--Semyon