Bug 318498 Many toolbars on top of one another
I don't know how to reproduce this. I'll attach a screenshot.
bundles/org.eclipse.e4.ui.workbench.renderers.swt/src/org/eclipse/e4/ui/workbench/renderers/swt/LazyStackRenderer.java
private EventHandler lazyLoader = new EventHandler() {
		public void handleEvent(Event event) {
			Object element = event.getProperty(UIEvents.EventTags.ELEMENT);
			if (!(element instanceof MGenericStack<?>))
				return;
			MGenericStack<MUIElement> stack = (MGenericStack<MUIElement>) element;
			LazyStackRenderer lsr = (LazyStackRenderer) stack.getRenderer();
			if (lsr == null)
				return;
			
			MUIElement oldSel = (MUIElement) event
					.getProperty(UIEvents.EventTags.OLD_VALUE);
			if (oldSel != null) {
				List<MUIElement> goingHidden = new ArrayList<MUIElement>();
				hideElementRecursive(oldSel, goingHidden);
			}
			if (stack.getSelectedElement() != null)
				lsr.showTab(stack.getSelectedElement());
		}
	};;
	public LazyStackRenderer() {
		super();
	}