Bug 282947 [CSS] ETabFolder does not draw right with border-visible: false
CTabFolders have always acted odd with respect to borders off and the first tab when selected. It just starts drawing top left. This behaviour seems to be confusing the ETabFolder draw code. It should draw the same curve regardless of whether borders are on or not.
bundles/org.eclipse.e4.ui.workbench.renderers.swt/src/org/eclipse/e4/workbench/ui/renderers/swt/StackModelFactory.java
&& policy.indexOf("EditorStack") >= 0; 
		if (ctf.getItemCount() == 0 && !isEditorStack) {
			parentElement.setVisible(false);
		}
	}
	@Override
	public void hookControllerLogic(final MPart<?> me) {
		super.hookControllerLogic(me);
		final MStack sm = (MStack) me;
		
		CTabFolder ctf = (CTabFolder) me.getWidget();
		ctf.addSelectionListener(new SelectionListener() {
			public void widgetDefaultSelected(SelectionEvent e) {
			}
			public void widgetSelected(SelectionEvent e) {
				MItemPart<?> newPart = (MItemPart<?>) e.item.getData(OWNING_ME);
				if (sm.getActiveChild() != newPart) {
					activate(newPart);
				}
				showTab(newPart);
			}
		});
		ctf.addCTabFolder2Listener(new CTabFolder2Adapter() {
			public void close(CTabFolderEvent event) {
				MPart part = (MPart) event.item.getData(PartFactory.OWNING_ME);
				part.setVisible(false);
			}
		});