Bug 304211 StackRenderer does not set eagerly created controls to the item
Right-clicking on a Java element and then using 'Open Declaration' opens an editor on the specified file but its content is not shown. Switching tabs or selecting items in the 'Outline' view does _not_ help. Seems to be calling IWorkbenchPage's APIs properly. No idea yet as to what's going on over here.
bundles/org.eclipse.e4.ui.workbench.renderers.swt/src/org/eclipse/e4/workbench/ui/renderers/swt/StackRenderer.java
ctf.addCTabFolder2Listener(closeListener);
		
		
		ctf.addListener(SWT.Activate, new org.eclipse.swt.widgets.Listener() {
			public void handleEvent(org.eclipse.swt.widgets.Event event) {
				CTabFolder ctf = (CTabFolder) event.widget;
				MPartStack stack = (MPartStack) ctf.getData(OWNING_ME);
				MPart part = stack.getSelectedElement();
				if (part != null)
					activate(part);
			}
		});
	}
	protected void showTab(MUIElement element) {
		super.showTab(element);
		CTabFolder ctf = (CTabFolder) getParentWidget(element);
		CTabItem cti = findItemForPart(element);
		if (element.getWidget() == null) {
			Control tabCtrl = (Control) renderer.createGui(element);
			cti.setControl(tabCtrl);
		}
		ignoreTabSelChanges = true;
		ctf.setSelection(cti);
		ignoreTabSelChanges = false;
		
		if (ctf.getTopRight() != null) {
			ctf.getTopRight().dispose();
			ctf.setTopRight(null);
		}
		ToolBar tb = getToolbar(element);