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According to memory leak tests a xml document is still held after all documents are closed in the ide, please see the org.netbeans.modules.editor.NbEditorDocument instance #9 from the heap dump for path to gc root. The heap dump is available at https://netbeans.org/projects/performance/downloads/download/heapdumps/Memory_leak_xml.7z
The document is held by XML navigator. The desired function is, however to see the last-edited file's outline in the navigator: so the relevant document is still referenced through the (visible) Navigator although all file's editors are closed.
*** Bug 235454 has been marked as a duplicate of this bug. ***