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
private List fTextConverters;
		private OverviewRuler fOverviewRuler;
		private boolean fIsOverviewRulerVisible;
		
		private AbstractHoverInformationControlManager fOverviewRulerHoveringController;
		private boolean fIgnoreTextConverters= false;
		
		private IVerticalRuler fCachedVerticalRuler;
		private boolean fCachedIsVerticalRulerVisible;
		
		
		public AdaptedSourceViewer(Composite parent, IVerticalRuler ruler, int styles) {
			super(parent, ruler, styles, CompilationUnitEditor.this);
			
			fCachedVerticalRuler= ruler;
			fCachedIsVerticalRulerVisible= (ruler != null);
			fOverviewRuler= new OverviewRuler(VERTICAL_RULER_WIDTH);
			
			delayedCreateControl(parent, styles);
		}