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[recent sources] In editor.structure/src/org/netbeans/modules/editor/structure/formatting/TagBasedLexerFormatter.java, an atomicLock is acquired, but the code has a path on which the lock will not be unlocked (if there is no TokenHierarchy for the Document). Although this probably should not happen in practice, it is a very bad practice, and should be fixed.
The module is no longer part of xml. It is maintained by Marek Fukala. And I do not know the appropriate component/sub component hence assigning to mfukala for his input.
Fixed, Change log: http://hg.netbeans.org/main/rev/9e88a0f84320