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While working on http://www.netbeans.org/issues/show_bug.cgi?id=98405, I see that the lastEditor field keeps a strong reference to the last editor, there by it leaks memory.
There are additional memory leaks from elsewhere. In order to narrow it down, I disabled xml code fold and navigation. Perhaps you should do the same. Take a look at xml/text-edit module's layer file.
Should be fixed now. *** This issue has been marked as a duplicate of 100752 ***