Created attachment 102279 [details]
(not sure where Maven went in the bug page's project list, sorry if this is misfiled)
After warm start, opened the POM file in the editor and modified its dependencies. Added one unit test class. Then realized I was changing the wrong project and invoked Mercurial > Revert in the project view, with the new test file visible in the editor.
Immediately got the attached deadlock - Swing document lock problem.
Well-known bug with fix in dev builds, I just do not have physical authority to push an update.
*** This bug has been marked as a duplicate of bug 187615 ***