Bug 284138 [CSS] ETabItem wrong background color behind X
If the close box is not shown (but space is allocated, e.g. editors) then the a gray background is drawn in it's place. It's the wrong color background though, it should be now the unselected tab background color.
bundles/org.eclipse.e4.ui.widgets/src/org/eclipse/e4/ui/widgets/ETabItem.java
Image image = parent.selectionBgImage;
			Color[] colors = parent.selectionGradientColors;
			int[] percents = parent.selectionGradientPercents;
			boolean vertical = parent.selectionGradientVertical;
			xx = x;
			yy = y + 1;
			ww = width;
			hh = height;
			parent.drawBackground(gc, shape, xx, yy, ww, hh, defaultBackground, image, colors, percents, vertical);
		}
	}
	
	shape[0] = Math.max(0, parent.borderLeft - 1);
	shape[shape.length - 2] = size.x - parent.borderRight + 1;
	RGB inside = parent.selectionBackground.getRGB();
	if (parent.selectionBgImage != null || 
	    (parent.selectionGradientColors != null && parent.selectionGradientColors.length > 1)) {
	    inside = null;
	}
	RGB outside = parent.getBackground().getRGB();		
	if (parent.gradientColors != null && parent.gradientColors.length > 1) {
	    outside = null;
	}
	
	Color borderColor = getETabParent().tabKeylineColor;
	parent.antialias(shape, borderColor.getRGB(), inside, outside, gc);
	gc.setForeground(borderColor);
	gc.drawPolyline(shape);
	
	if (!tabInPaint) return;