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
Control control= getControl();
			if (control instanceof Composite) {
				Composite composite= (Composite) control;
				composite.setLayout(new AdaptedRulerLayout(GAP_SIZE, this));
				fOverviewRuler.createControl(composite, this);
			}
		}
		
		private void ensureOverviewHoverManagerInstalled() {
			if (fOverviewRulerHoveringController == null && fAnnotationHover != null && fHoverControlCreator != null)	{
				fOverviewRulerHoveringController= new OverviewRulerHoverManager(fOverviewRuler, this, fAnnotationHover, fHoverControlCreator);
				fOverviewRulerHoveringController.install(fOverviewRuler.getControl());
			}
		}
		
		public void hideOverviewRuler() {
			fIsOverviewRulerVisible= false;
			Control control= getControl();
			if (control instanceof Composite) {
				Composite composite= (Composite) control;
				composite.layout();
			}
			if (fOverviewRulerHoveringController != null) {
				fOverviewRulerHoveringController.dispose();
				fOverviewRulerHoveringController= null;
			}
		}
		
		public void showOverviewRuler() {