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Created attachment 134418 [details] stacktrace see attached stacktrace, sometimes we read project files from EDT which may lead to huge UI freeze
The file contents are read due to request of file's mime type in order to properly determine an editor kit to be used for file's editing. IMHO we could attempt to find the kit itself during CloneableEditor.initNonVisual(). pane.setEditorKit(kit) call must then certainly be done from AWT as well as pane.setDocument().