Bug 28698 Ctrl+O (show outline) does not work if the file being shown in the editor is read only
M4 The summary tells all. Works if the file is not read only. Is this by design?
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/CompilationUnitEditor.java
fIsOverviewRulerVisible= true;
			Control control= getControl();
			if (control instanceof Composite) {
				Composite composite= (Composite) control;
				composite.layout();
			}
			ensureOverviewHoverManagerInstalled();
		}
		
		public boolean isOverviewRulerVisible() {
			return fIsOverviewRulerVisible;
		}
		
		/*
		 * @see ISourceViewer#setDocument(IDocument, IAnnotationModel, int, int)
		 */
		public void setDocument(IDocument document, IAnnotationModel annotationModel, int visibleRegionOffset, int visibleRegionLength) {
			super.setDocument(document, annotationModel, visibleRegionOffset, visibleRegionLength);
			fOverviewRuler.setModel(annotationModel);
		}
		
		
		public void updateIndentationPrefixes() {
			SourceViewerConfiguration configuration= getSourceViewerConfiguration();
			String[] types= configuration.getConfiguredContentTypes(this);
			for (int i= 0; i < types.length; i++) {
				String[] prefixes= configuration.getIndentPrefixes(this, types[i]);
				if (prefixes != null && prefixes.length > 0)
					setIndentPrefixes(prefixes, types[i]);
			}