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
tb = (ToolBar) createToolBar(part.getParent(), ctf, tbModel);
		} else {
			tb = new ToolBar(ctf, SWT.FLAT | SWT.HORIZONTAL);
		}
		
		if (part.getMenu() != null) {
			addMenuButton(part, tb, part.getMenu());
		}
		tb.pack();
		return tb;
	}
	/**
	 * @param tb
	 */
	private void addMenuButton(MItemPart<?> part, ToolBar tb, MMenu menu) {
		ToolItem ti = new ToolItem(tb, SWT.PUSH);
		ti.setImage(getViewMenuImage());
		ti.setHotImage(null);
		ti.setToolTipText("View Menu"); 
		ti.setData("theMenu", menu); 
		ti.setData("thePart", part); 
		ti.addSelectionListener(new SelectionListener() {
			public void widgetSelected(SelectionEvent e) {
				showMenu((ToolItem) e.widget);
			}
			public void widgetDefaultSelected(SelectionEvent e) {
				showMenu((ToolItem) e.widget);
			}
		});
	}